EP1163201A1 - Method for producing acrylic acid - Google Patents

Method for producing acrylic acid

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Publication number
EP1163201A1
EP1163201A1 EP00907643A EP00907643A EP1163201A1 EP 1163201 A1 EP1163201 A1 EP 1163201A1 EP 00907643 A EP00907643 A EP 00907643A EP 00907643 A EP00907643 A EP 00907643A EP 1163201 A1 EP1163201 A1 EP 1163201A1
Authority
EP
European Patent Office
Prior art keywords
acrylic acid
column
weight
liquid
cleavage
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP00907643A
Other languages
German (de)
French (fr)
Other versions
EP1163201B1 (en
Inventor
Jürgen Schröder
Gerhard Nestler
Klaus Joachim MÜLLER-ENGEL
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
BASF SE
Original Assignee
BASF SE
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from DE1999109923 external-priority patent/DE19909923A1/en
Priority claimed from DE1999124533 external-priority patent/DE19924533A1/en
Application filed by BASF SE filed Critical BASF SE
Publication of EP1163201A1 publication Critical patent/EP1163201A1/en
Application granted granted Critical
Publication of EP1163201B1 publication Critical patent/EP1163201B1/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42—Separation; Purification; Stabilisation; Use of additives
    • C07C51/487—Separation; Purification; Stabilisation; Use of additives by treatment giving rise to chemical modification
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D3/00—Distillation or related exchange processes in which liquids are contacted with gaseous media, e.g. stripping
    • B01D3/14—Fractional distillation or use of a fractionation or rectification column
    • B01D3/16—Fractionating columns in which vapour bubbles through liquid
    • B01D3/22—Fractionating columns in which vapour bubbles through liquid with horizontal sieve plates or grids; Construction of sieve plates or grids
    • B01D3/225—Dual-flow sieve trays
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42—Separation; Purification; Stabilisation; Use of additives
    • C07C51/43—Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42—Separation; Purification; Stabilisation; Use of additives
    • C07C51/43—Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation
    • C07C51/44—Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation by distillation
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/42—Separation; Purification; Stabilisation; Use of additives
    • C07C51/50—Use of additives, e.g. for stabilisation

Definitions

  • the present invention relates to a process for the preparation of acrylic acid, in which a heterogeneously catalyzed gas phase partial oxidation of propane or propene and / or acrolein with molecular oxygen on catalysts in the solid state of aggregation produces a hot product gas mixture containing acrylic acid at elevated temperature, the temperature of the hot , Acrylic acid-containing product gas mixture reduced by direct cooling with a quench liquid, the non-evaporating part of the quench liquid 1, optionally via an indirectly cooling heat exchanger, circulates and discharges part of the non-evaporated quench liquid 1 from this circuit as an outlet, then the cooled product gas mixture conducts into a column equipped with separating internals, can rise within itself in the column and condenses fractionally, with a crude acrylic acid and over in the side draw the column bottoms and / or a bottoms liquid and / or high boiler fraction containing acrylic acid oligomers located near the bottom and removed and used as quench liquid 1.
  • Acrylic acid is an important intermediate product which is used, for example, in the production of polymer dispersions.
  • acrylic acid is obtainable by heterogeneously catalyzed gas phase partial oxidation of propane or propene and / or acrolein with molecular oxygen on catalysts in the solid state at elevated temperature.
  • the starting gases mentioned usually with inert gases such as Diluted nitrogen, CO and / or water vapor, passed in a mixture with molecular oxygen at elevated temperatures and, if appropriate, increased pressure over transition-metal mixed oxide catalysts and converted oxidatively into a product gas mixture containing acrylic acid.
  • a basic separation of acrylic acid from the product gas stream can be achieved by absorption in a suitable absorption medium, e.g. water or a mixture of 70 to 75% by weight diphenyl ether and 25 to 30% by weight diphenyl (cf. e.g. EP-A 297445 and DE-PS 2136396).
  • a suitable absorption medium e.g. water or a mixture of 70 to 75% by weight diphenyl ether and 25 to 30% by weight diphenyl (cf. e.g. EP-A 297445 and DE-PS 2136396).
  • DE-A 19740253, DE-A 19740252, DE-A's 19814421, 19814387 and 19814375 disclose a process for the preparation of acrylic acid as described in the introduction, in which the basic separation of the crude acrylic acid by fractional condensation of the product gas mixture of the heterogeneously catalyzed gas phase Partial oxidation is carried out. Such a procedure is also the subject of DE-A 19909923.
  • acrylic acid in the condensed phase forms Michael addition to itself and to the acrylic acid dimer which forms in the process, acrylic acid oligomers (Michael adducts) (the term acrylic acid oligomers means in this document always the corresponding Michael adducts and acrylic acid oligomers not formed by radical polymerization).
  • acrylic acid oligomers means in this document always the corresponding Michael adducts and acrylic acid oligomers not formed by radical polymerization.
  • acrylic acid oligomers Since acrylic acid oligomers have a higher boiling temperature than acrylic acid, they accumulate in the bottom liquid both in the course of a distillative • separation of acrylic acid and in the case of fractional condensation of the product gas mixture of a gas-phase catalytically oxidative acrylic acid production. In principle, the formation of acrylic acid oligomers is undesirable because it reduces the yield of valuable product.
  • This thermal cleavage can be catalyzed both acidic and basic or carried out in the absence of catalysts (see e.g. EP-A 717031).
  • the object of the present invention was to split back the process in the process for the production of acrylic acid as described in the introduction, in which the basic separation of the crude acrylic acid is carried out by fractional condensation of the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation contained acrylic acid oligomers so that the running time of the process, in particular that of the column for fractional condensation, is essentially not reduced.
  • a process for the preparation of acrylic acid was produced in which a hot product-gas mixture containing acrylic acid is produced at elevated temperature by heterogeneously catalyzed gas-phase partial oxidation of propane or propene and / or acrolein with molecular oxygen on catalysts in the solid state of aggregation, the temperature of hot, acrylic acid-containing product gas mixture reduced by direct cooling with a quench liquid 1, the non-evaporating part of the quench liquid 1, optionally via an indirectly cooling heat exchanger, is circulated and a part of the non-evaporated quench liquid 1 is discharged from this circuit as an outlet, then the cooled product gas mixture leads into a column equipped with separating internals, can rise within itself in the column and condenses fractionally, with a crude acrylic acid in the side draw and over the column sump and / or sump liquid and / or high boiler fraction (less volatile than
  • Acrylic acid is removed and used as quench liquid 1, which is characterized in that the outlet of the Feeds quench liquid 1 to a cracking vessel and in the same the acrylic acid oligomers contained in the outlet of quench liquid 1 are split back into acrylic acid at elevated temperature, acrylic acid escaping gaseously from the liquid phase is condensed 5 and the resulting condensate is fed into the circuit of quench liquid 1.
  • fractional condensation within the column can be effected in a manner known per se by indirect cooling and / or heating. However, it is more convenient to use fractional condensation
  • the crude acrylic acid which normally has a purity> 95% by weight, is taken off in the side draw as a medium boiler fraction via the second collecting tray.
  • the crude acrylic acid is expediently used for further distillative and / or crystallizing purification stages
  • Low-boiling gas flow is expediently obtained by direct cooling in a space free of or containing internals by means of a second quench liquid (referred to in this document for the purpose of differentiation as quench liquid 2)
  • Condensate 45 substantial water and components that are more volatile than water.
  • the condensate obtained is referred to as sour water.
  • Part of the acid water is in sensible way to increase the separation performance at the top of the column in the same.
  • a further part of the acid water is expediently discharged and disposed of (for example burned) and the remaining part of the acid water is usually cooled indirectly in an external heat exchanger and used as quench liquid 2.
  • Components of the low-boiling stream which are more volatile than water are drawn off essentially in gaseous form and, if appropriate, are returned to the gas phase oxidation as a diluent gas.
  • the acid water quench can be integrated into the column for the fractional condensation.
  • aqueous reflux liquid is withdrawn via a further collecting tray in the upper part of the column, indirectly cooled in a heat exchanger and partly returned to the top of the column and partly below the collecting tray except for the outlet portion to be disposed of.
  • the waste gas which is possibly to be returned to the gas phase oxidation leaves the column at the top thereof.
  • the quenching of the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation with the quenching liquid 1 is generally carried out in a free space in internals.
  • the quench liquid 1 which reaches the cooling limit temperature is circulated, if necessary via an external heat exchanger, and reused for direct cooling. Part of the quench liquid 1 is discharged from this circuit as an outlet and fed to the cleavage to be integrated according to the invention.
  • the degree of purity of the crude acrylic acid withdrawn from the fractionated condensation column via side draw is usually> 95% by weight. That is, the crude acrylic acid contains at least 95% of its weight acrylic acid molecules.
  • the polymerization inhibition of the process according to the invention can be carried out essentially as described in DE-A 19909923. This means that it can be carried out, for example, by adding phenothiazine or a mixture of an N-oxyl radical and a benzene compound which has two substituents bonded to the aromatic ring via heteroatoms and at least one mobile hydrogen at the top of the condensation column.
  • a more favorable form of polymerization inhibition is characterized in that it takes place exclusively by means of N-oxyl radicals (for example those mentioned in EP-A 765856). These are compounds that have at least one group -NO «.
  • N-oxyl radicals are the pyrrolidine-1-oxyl types and the piperidine-1 -oxy types.
  • examples include 4, 4 ', 4 "-Tris- (2, 2, 6, 6 - tetramethylpiperidine-l-oxyl) phosphite, 2,2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPO), 4 -Hydroxy- 2, 2, 6, 6-tetramethylpiperidine-1 -oxyl (4-OH-TEMPO),
  • the N-oxyl inhibitors are preferably 0.1 to 2 wt. -% solutions used in water and / or in acid water.
  • the aqueous N-oxyl inhibitor solution is expediently added in the upper quarter of the condensation column and / or in the acid water quench.
  • the aqueous solution can contain only one N-oxyl radical or a mixture of N-oxyl radicals. Adequate inhibition can already be achieved by adding an aqueous solution which contains only 4-OH-TEMPO as a polymerization inhibitor.
  • the addition amount of the N-oxyl inhibitors to be used is expediently such that the high boiler fraction and bottom liquid removed from the column contain 1 to 1000 ppm by weight, based on the weight of the high boiler fraction, of N-oxyl inhibitors. Since the high boiler fraction and bottom liquid removed from the column serve as quench liquid 1, the quench system 1 is automatically stabilized as well. If necessary, the quench system 1 can be stabilized by adding a phenothiazine compound. Examples of such phenothiazine compounds include phenothiazine (PTZ) itself,
  • phenothiazine Bis- ( ⁇ -methylbenzyl) phenothiazine, 3, 7 -dioctylphenothiazine and bis- ( ⁇ -dimethylbenzyl) phenothiazine, among which phenothiazine is preferred.
  • the latter applies in particular when 4-OH-TEMPO is used for the stabilization of the condensation column or is used exclusively.
  • Such an addition of phenothiazine based on the weight of the quench liquid, can be 1 to 500 ppm by weight.
  • a phenothiazine compound is expediently added in acrylic acid, preferably dissolved in crude acrylic acid taken off via a side draw (typically 0.1 to 2% by weight).
  • the need for N-oxyl radicals required to inhibit polymerization can be reduced in that the less demanding stabilization of acidic water, ie the stabilization of the quench system 2, alternatively or in combination with an aqueous solution of at least one benzene compound, the two via heteroatoms on the aromatic ring has bound substituents and at least one mobile hydrogen (for example those mentioned in EP-A 766856), for example a phenol compound, is carried out (typically 0.1 to 2% by weight solution).
  • 1 to 500 ppm by weight, based on the weight of the acid water, of at least one phenol compound are used for such acid water inhibition.
  • An alternative polymerization inhibition is to add an aqueous solution of MEHQ to the top of the condensation column and to carry out the acid water inhibition likewise by adding a solution of MEHQ in water and / or in acid water.
  • a solution of PTZ in acrylic acid e.g. crude acrylic acid
  • Qench 1 a solution of PTZ in acrylic acid (e.g. crude acrylic acid) is introduced in the middle section of the condensation column and, if appropriate, in the product gas mixture quench (Quench 1).
  • the quench system 1 is automatically stabilized via the stabilization of the condensation column and can be co-stabilized if necessary by adding phenothiazine and / or metoquinone.
  • the polymerization inhibition of the quench liquid 1 achieved in this way is generally also sufficient to ensure sufficient stability of the outlet of the quench liquid 1 against undesired during the cleavage to be integrated according to the invention
  • the condenser surfaces are expediently additionally inhibited during the condensation of the vapors escaping in the course of the splitting-off, containing acrylic acid (the inhibitors contained in the quenching liquid 1 generally do not also evaporate).
  • This polymerization inhibition of the condenser surfaces will advantageously be carried out using the same polymerization inhibitors which were also used and / or recommended for inhibiting the condensation column. If, for example, only 4 -OH-EMPO is used to stabilize the condensation column and the acid water quench, it makes sense to carry out the condenser surfaces exclusively with 4-OH-TEMPO, appropriately dissolved in crude acrylic acid.
  • the capacitor surfaces can also be stabilized by means of PTZ, MEHQ and / or hydroquinone or only stabilized.
  • the cleavage to be integrated according to the invention is carried out at a pressure of ⁇ 1 bar and at a temperature of 130 to 250 ° C.
  • the pressure for the cleavage is advantageously 25 to 600, preferably 100 to 300 mbar.
  • the cleavage temperature is expediently from 140 to 230 ° C., preferably from 160 to 200 ° C. If the cleavage is carried out continuously (the process according to the invention is preferably carried out continuously), the residence time in the cleavage reactor should be about 0.5 to 3 h.
  • the cleavage to be integrated according to the invention can be carried out in a simple manner in a heatable stirred reactor.
  • the cleavage of the acrylic acid oligomers contained in the outlet of the quench system 1 can be carried out without the addition of special acidic or basic cleavage catalysts.
  • the cleavage will advantageously be carried out in the presence of cleavage catalysts.
  • cleavage catalysts e.g. Dodecylbenzenesulfonic acid, p-toluenesulfonic acid, sulfuric acid or the solid acid catalysts of JP-A 3-178949.
  • Polymerization inhibitor or used as co-stabilizer of the quench system 1 it is expedient to carry out the cleavage by adding an inorganic salt, the addition of which to an aqueous solution of a strong Bronsted acid shifts the pH of the aqueous solution to alkaline, as is the case, for example DE-C 2407236 recommends.
  • the amount of basic cleavage catalyst to be added will generally be 0.1 to 5% by weight.
  • cleavage catalysts suitable according to the invention are KOH, K 2 C0 3 , KHC0 3 , NaOH, Na 2 C0 3 , NaHC0 3 , LiOH, Li 2 C0 3 and CaC0 3 .
  • suitable cleavage catalysts are, in particular, the alkali and / or alkaline earth metal salts of weak inorganic or organic Bronsted acids such as, for example, phosphoric acid, boric acid, carbonic acid, formic acid or acetic acid.
  • alkali and / or are particularly suitable as cleavage catalysts Alkaline earth metal phosphates, borates, carbonates, bicarbonates, formates and acetates.
  • the cleavage catalysts will preferably be selected so that they are soluble in the outlet of the quench liquid 1 under the cleavage conditions. According to US-A 4293347, the presence of dialkyl phthalates also has an advantageous effect on the relevant cleavage.
  • the hardly volatile residue remaining in the cleavage reactor is regularly disposed of in the process according to the invention, e.g. its combustion.
  • the cleavage residue can optionally be diluted with hydrophilic organic liquids such as organic acids (e.g. ethylhexanoic acid, propionic acid) or residues from the production of these acids, but also with alkanols such as ethanol or methanol.
  • the quench liquid 1 to be used in the process according to the invention can, if necessary, be added with an inert organic liquid which boils higher than acrylic acid and which keeps the quench liquid 1 fluid.
  • Such high-boiling inert organic liquids are in particular all those which are recommended in DE-A 2136396 and in DE-A 4308087. These are essentially liquids whose boiling point at normal pressure is above 160 ° C. Examples include ethylhexanoic acid, N-methylpyrrolidone, middle oil fractions from paraffin distillation, diphenyl ether, diphenyl or mixtures of the aforementioned liquids, such as a mixture of 70 to 75 wt .-% diphenyl ether and 25 to 30 wt .-% diphenyl.
  • the inert organic liquid which is also used is also evaporated during the cleavage. If a portion of the organic liquid remains in the cleavage residue, it can be subjected to workup in which the solvent used, e.g. distilled, separated and recycled to quench 1. The remaining high boilers are disposed of.
  • the hot product gas mixtures of the heterogeneously catalyzed gas phase oxidation are often composed as follows:
  • maleic acid / maleic anhydride 0.01 to 0.5% by weight of maleic acid / maleic anhydride, 0.05 to 1% by weight of acrolein, 0.05 to 1% by weight of formaldehyde, 0.1 to 1% by weight of furfural, 0.01 up to 0.5% by weight of benzaldehyde, 0.01 to 1% by weight of propene, 0.05 to 10% by weight of oxygen, 1 to 30% by weight of water and
  • inert gases such as Nitrogen, carbon oxides, methane and propane.
  • the gas phase oxidation itself can e.g. be carried out in two successive oxidation stages, as described in EP-A 700 714 and in EP-A 700 893.
  • the gas phase oxidations cited in DE-A 19740253 and in DE-A 19740252 can also be used.
  • the temperature of the product gas mixture leaving the gas phase oxidation is 150 to 350 ° C., usually 200 to 300 ° C.
  • the hot product gas mixture is usually cooled to a temperature of 100 to 180 ° C., at which it is then led into the lowest section of the separation column.
  • the operating pressure in the column is generally 0.5 to 5 bar, often 0.5 to 3 bar and often 0.5 to 2 bar.
  • All common heat exchangers or heat exchangers are suitable for indirect cooling or heating of the quench liquid 1.
  • Pipe bundle heat exchangers, plate heat exchangers and air coolers may be mentioned as preferred.
  • the temperature of the quench liquid 1 is normally 70 to after leaving the heat exchanger
  • Suitable cooling media are air for the corresponding air cooler and cooling liquids, in particular water, for the other cooling devices.
  • all common internals come into consideration as column internals, in particular trays, packings and / or packing elements. Of the floors are
  • the Total number of separating plates 20 to 80 in the event that the acid water quench is integrated in the column up to 100, preferably 50 to 80 (or 100).
  • the crude acrylic acid removed in the side hood usually still contains
  • the temperature in the bottom of the column is typically 90 to 130 ° C, whereas the top temperature is normally 40 to 80, often 50 to 70 ° C.
  • the extraction temperature of the crude acrylic acid is usually 80 to 110 ° C.
  • the return temperature of the high boilers when entering the column is typically 95 to 115 ° C.
  • the return temperature of the acid water to the column is usually 25 to 35 ° C.
  • the quench system 2 can be designed like the quench system 1.
  • the crude acrylic acid can also be removed via a plurality of collecting trays which follow one another at short intervals.
  • the crude acrylic acid removed as a medium boiler fraction can be used for further purification, e.g. be supplied to a crystallization.
  • This is usually carried out without the addition of a solvent, in particular without the addition of an organic solvent.
  • the crystallization process to be used is not restricted.
  • the crystallization can be carried out continuously or discontinuously, in one or more stages up to almost any degree of purity.
  • water can be added to the crude acrylic acid to be crystallisatively purified beforehand (based on the amount of acrylic acid up to 10% by weight or more, preferably up to 5% by weight).
  • Such an addition facilitates the removal of acetic acid contained as a by-product in the crude acrylic acid, since this is incorporated to a lesser extent into the acrylic acid crystals in the presence of water.
  • the presence of water reduces the tendency to crust.
  • crude acrylic acid suitable for esterification can be achieved with a single crystallization step.
  • This crystallization stage is expediently carried out as a suspension crystallization.
  • a trough in which wiped cooling plates (through which a cooling medium flows internally) is advantageously used as the crystallizer. are arranged hanging from each other. Wiping the cooling plates suppresses the formation of layer crystallization.
  • the crude acrylic acid is continuously fed through the crystallizer from the back to the front (pumped or overflow controlled).
  • the single-phase crude acrylic acid thickened to a two-phase
  • Acrylic acid crystals as a solid phase containing suspension is a crystallizer that is particularly suitable in this regard.
  • a crystallizer that is particularly suitable in this regard is a crystallizer from GMF Gouda (Holland) of the type cooling disc crystallizer.
  • the crystals formed are expediently separated from the abovementioned suspension by means of a centrifuge (for example one from Siebtechnik, of the SHS type centrifugal centrifuge with conical sieve drum) and, if necessary, washed with crude acrylic acid which has already been further purified and / or sweating described later subject.
  • the separation and washing of the suspension crystals can, however, also advantageously be carried out in a washing column, as it is e.g.
  • layer crystallization e.g. falling film crystallization, as used e.g. is described in EP-A 616 998 for obtaining pure acrylic acid.
  • the liquid coolant / heat transfer medium is e.g. Water / methanol, water / ethanol and water / ethylene glycol mixtures are considered.
  • the crystallization is expediently carried out as a fractional crystallization.
  • all stages which produce a crystallizate which is purer than the supplied melt are usually called cleaning stages. All other stages are called output stages.
  • Appropriately - multi-stage processes are operated according to the countercurrent principle, in which after crystallization the crystals are separated from the mother liquor in each stage and this crystals are added to the respective stage with the next highest degree of purity is carried out while the crystallization residue is fed to the respective stage with the next lowest degree of purity.
  • the temperature of the liquid phase during the crystallization is advantageously between +5 and + 14 ° C, in particular between + 8 ° C and + 12 ° C.
  • the solids content in the crystallizer is advantageously between 0 and 80 g solids / 100 g total mass. In the case of suspension crystallization, the solids content is preferably between 15 and 35 g of solids / 100 g of total mass and in the case of layer crystallization 50 to 80 g of solids / 100 g of total mass.
  • the crystallization takes place by cooling the apparatus walls and / or by evaporating the solution in vacuo.
  • the heat is dissipated via scratch coolers which are connected to a stirred kettle or a container without a stirrer. The circulation of the crystal suspension is guaranteed by a pump.
  • Another embodiment in cooling crystallization is the use of cooling disk crystallizers, such as are used e.g. manufactured by Gouda (Holland).
  • the heat is transferred via conventional heat exchangers (preferably tube bundle or
  • the crystallization takes place in apparatuses in which the crystals grow on cooled surfaces in the crystallization apparatus, ie are fixed in the apparatus (for example layer crystallization processes (cf. for example EP-A 616998) from Sulzer Chemtech (Switzerland) or static crystallization process (see, for example, FR-A 2668946) from BEFS PROKEM (France)).
  • layer crystallization processes cf. for example EP-A 616998
  • Sulzer Chemtech Switzerland
  • static crystallization process see, for example, FR-A 2668946 from BEFS PROKEM (France)
  • the acrylic acid crystals obtained are, as already mentioned, separated from the remaining mother liquor.
  • layer crystallization or static crystallization the separation of the crystals from the mother liquor in the crystals! - on the apparatus itself, since the crystals are fixed in the apparatus and the mother liquor can be removed from the apparatus by draining off.
  • the crystals are removed from the crystallizer by melting the crystals and then allowing the melt to flow away.
  • suspension crystallization all known methods of solid-liquid separation are suitable.
  • the crystals are separated from the mother liquor by filtering and / or centrifuging.
  • the filtering or centrifuging is advantageously preceded by a pre-thickening of the suspension, for example by means of hydrocyclone (s). All known centrifuges that work discontinuously or continuously are suitable for centrifugation. Shear centrifuges which can be operated in one or more stages are most advantageously used.
  • screw screen centrifuges or screw discharge centrifuges (decanters) are also suitable. Filtration is advantageously carried out by means of suction filters, which are operated batchwise or continuously, with or without an agitator, or by means of a belt filter. In general, the filtering can be carried out under pressure or in vacuo.
  • the crystals or the crystal cake are washed and / or sweated in one or more stages.
  • the amount of washing liquid is suitably between 0 and 500 g of washing liquid / 100 g of crystals, preferably between 30 and 200 g of washing liquid / 100 g of crystals.
  • the washing liquid used is not subject to any restrictions. However, it is advantageous to wash with pure product, i.e. with a liquid containing acrylic acid, the purity of which is equal to or higher than that of the crystal cake to be washed. Washing with water is also possible. Washing can be carried out in the usual apparatus for this.
  • Washing columns are advantageous in which the separation of the mother liquor and the washing take place in one apparatus, centrifuges which operate in one or more stages can be used, or filter suction or band filter. Washing can be carried out on centrifuges or belt filters in one or more stages. Here, the washing liquid can be passed in countercurrent to the crystal cake.
  • Sweating is a local melting of contaminated areas.
  • the amount of sweat is advantageously between 0 and 80 g of melted crystals / 100 g of crystals before sweating, preferably between 5 and 35 g of melted crystals / 100 g of crystals. It is particularly preferred to carry out the sweating on centrifuges or belt filters and in crystallizers in which the crystals are fixed in the crystallizer (e.g. layer crystallizer). Carrying out a combination of washing and sweating in one apparatus can also be suitable.
  • the acrylic acid crystals after the solid-liquid separation and possibly further washing and / or sweating represent the purified acid from the process.
  • the purity of the crystals obtained is generally 97 to 99.99% by weight or more acrylic acid , in particular 98.5 to 99.9% by weight of acrylic acid.
  • the crystals produced by the process according to the invention now contain only very small amounts of impurities, such as acetic acid, propionic acid and / or diacrylic acid.
  • the crystallization according to the invention can in principle be carried out as suspension crystallization and / or as layer crystallization (the deposited crystals remain fixed in the crystallizer).
  • the latter crystallization method comes e.g. falling film crystallization (e.g. as described in EP-A 616 998), crystallization in a fully flow-through tube (e.g. according to DE-A 2606364) or static crystallization.
  • the cooling can be redirected directly (e.g. evaporation in a vacuum) and / or indirectly using cooled surfaces.
  • Possible crystallizers for such a suspension crystallization are: stirred tanks with wall-mounted stirrers, scratch coolers, cooling disk crystallizers from Gouda, and circulation crystallization with heat exchangers without a device for avoiding crystal layer formation.
  • the purified acid can be esterified by known methods or further purified by known methods.
  • the mother liquor which remains after the crystallization has ended is, at least in part, recycled into the separation column, as described at the outset.
  • the proportion of the recirculated mother liquor, based on its amount, is> 0 to, according to the invention
  • molecular oxygen or an inert gas stream containing molecular oxygen can be passed through the separation column in an accompanying manner. This enhances the effect of the polymerization inhibitors added.
  • Possible starting compounds for the gas phase oxidation are isobutene, methyl tert-butyl ether, isobutane, isobutyric acid, tert. Butanol, isobutyraldehyde or methacrolein. Otherwise, the information given in DE-A 19740253 and DE-A 19740252 apply in this regard.
  • the polymerization inhibition in the process according to the invention can also be carried out by adding an aqueous MEHQ solution to the top of the column and adding a solution of PTZ in acrylic acid to the middle part of the column.
  • the acidic water quench is then also stabilized using an aqueous MEHQ solution.
  • the product gas mixture (3600 g / h) was cooled to a temperature of 136 ° C. in a spray cooler (2).
  • the spray liquid (quench liquid 1) was 750 g / h (7) out of a total of 7000 g / h via the collecting tray (5) (with a temperature of 100 ° C.) from the
  • the product gas mixture cooled to a temperature of 136 ° C. was fed to the separation column (10) below the tray (5).
  • the column was a plate column with 25 dual-flow plates and 50 bell plates (1 bell per plate). The bottom diameter was 49 mm.
  • the dual-flow floors had 6 holes per floor.
  • the hole diameter of the first five dual-flow trays was 9.5 mm.
  • the subsequent 10 trays had a hole diameter of 9 mm and the hole diameter of the last 5 dual-flow trays was 8.7 mm.
  • the floor above floor 15 was designed as another collecting floor (11). 1800 g / h of crude acrylic acid (12) having a temperature of 97 ° C. were contained above it Acrylic acid 97.3% by weight
  • the crystallizer was a stirred tank (3 1 internal volume) with a spiral stirrer. The heat of crystallization was removed via the double jacket of the container. The equilibrium temperature of the solution was 9.7 ° C.
  • the suspension produced during the crystallization (solids content approx. 25% by weight) was discontinuously separated into crystals and mother liquor on a centrifuge at 2000 rpm (centrifuge diameter 300 mm) and a centrifuging time of 3 min. The crystals were then washed with melted (previously washed) crystals (80 g) for 20 seconds at 2000 rpm.
  • a gaseous mixture (17) was removed from the top of the column and subjected to a partial condensation in the spray cooler (18).
  • 480 g / h of the resulting acid water were returned to the top of the column at a temperature of 30 ° C. (26) - 220 g / h of the acid water were continuously removed.
  • 90 g / h of the sour water removed were mixed with MEHQ (22) and as 0.5 wt. -% aqueous stabilizer solution (21) together with the remaining amount of acid water (23) cooled to 18 ° C as a spray liquid via the water-cooled tube bundle heat exchanger (24) speed (25) used.
  • the separation device described could be operated for 40 days without any appreciable polymer formation.
  • Example 2 The procedure was as in Example 1, but instead of 1% by weight NaCO 3 , 1% by weight dodecylbenzenesulfonic acid was used as the cracking catalyst. 495 g of cleavage product of a composition similar to that in Example 1 were condensed within 4.75 h. This corresponds to a recovery rate of approx. 61% by weight.
  • a circulation reactor (29) consisting of glass, which was heated with a heating plug, was filled with 250 g of the outlet of the quench liquid 1 from the comparative example and heated.
  • Additive - loaned to the 4 -OH-TEMPO contained in the 250 g outlet of quench liquid 1, 2000 wt.ppm phenothiazine were added for the purpose of co-stabilization.
  • the gap temperature was 182 ° C and the gap pressure was 500 mbar.
  • a gap catalyst (32) was not added. With level control, 155 g / h of quench liquid 1 (9) were fed to the circulating reactor filled as described.
  • the cleavage products were separated off in vapor form via a splash guard attached to the reactor and condensed in an amount of 113 g / h (this corresponds to a recovery rate of 73% by weight) without addition of a polymerization inhibitor (30).
  • the condensate also consisted of approximately 3.2% by weight of diacrylic acid, 0.06% by weight of benzaldehyde, 0.3% by weight of acetic acid and 0.03% by weight of propionic acid.
  • the spray liquid was additionally supplied with 30 g / h of the cleavage condensate from Example 3 which had cooled to 100 ° C. (the addition point was just before the spray nozzle). In a corresponding manner, the amount of pure crystals obtained was approximately 400 g / h.
  • the recycling of the split condensate had no visible influence on the polymer formation within 40 days of operation of the separation device described.
  • the location marked with R in FIG. 2 can also be considered as an alternative addition point for the reclassification condensate.

Abstract

The invention relates to a method for producing acrylic acid in which a product gas mixture which can be obtained by the catalytic gas phase oxidation of a C3 precursor of the acrylic acid and which contains acrylic acid. After a direct cooling using a quenching liquid, said product gas mixture is fractionally condensed in a rectifying column, said column being provided with interior components, while ascending and with lateral discharge of a crude acrylic acid. The acrylic acid oligomers formed thereby are reversibly cleaved and returned.

Description

Verfahren zur Herstellung von AcrylsäureProcess for the production of acrylic acid
Beschreibungdescription
Vorliegende Erfindung betrifft ein Verfahren zur Herstellung von Acrylsäure, bei dem man durch heterogen katalysierte Gasphasen- Partialoxidation von Propan oder Propen und/oder Acrolein mit molekularem Sauerstoff an im festen Aggregatzustand befindlichen Katalysatoren bei erhöhter Temperatur ein Acrylsäure enthaltendes heißes Produktgasgemisch erzeugt, die Temperatur des heißen, Acrylsäure enthaltenden Produktgasgemisches durch direkte Kühlung mit einer Quenchflüssigkeit verringert, den dabei nicht verdampfenden Teil der Quenchflüssigkeit 1, gegebenenfalls über einen indirekt kühlenden Wärmetauscher, im Kreis führt und als Auslaß einen Teil der nicht verdampften Quenchflüssigkeit 1 aus diesem Kreislauf ausschleust, anschließend das abgekühlte Produktgasgemisch in eine mit trennwirksamen Einbauten ausgerüstete Kolonne leitet, innerhalb der Kolonne in sich selbst aufsteigen läßt und dabei fraktioniert kondensiert, wobei im Seitenabzug eine rohe Acrylsäure und über den Kolonnensumpf und/oder über einen in Sumpfnähe gelegenen Seitenabzug Acrylsäure-Oligomere enthaltende Sumpfflüssigkeit und/oder Schwersiederfraktion entnommen und als Quenchflüssigkeit 1 verwendet wird.The present invention relates to a process for the preparation of acrylic acid, in which a heterogeneously catalyzed gas phase partial oxidation of propane or propene and / or acrolein with molecular oxygen on catalysts in the solid state of aggregation produces a hot product gas mixture containing acrylic acid at elevated temperature, the temperature of the hot , Acrylic acid-containing product gas mixture reduced by direct cooling with a quench liquid, the non-evaporating part of the quench liquid 1, optionally via an indirectly cooling heat exchanger, circulates and discharges part of the non-evaporated quench liquid 1 from this circuit as an outlet, then the cooled product gas mixture conducts into a column equipped with separating internals, can rise within itself in the column and condenses fractionally, with a crude acrylic acid and over in the side draw the column bottoms and / or a bottoms liquid and / or high boiler fraction containing acrylic acid oligomers located near the bottom and removed and used as quench liquid 1.
Acrylsäure ist ein bedeutendes Zwischenprodukt, das beispielsweise im Rahmen der Herstellung von Polymerisatdispersionen Verwendung findet.Acrylic acid is an important intermediate product which is used, for example, in the production of polymer dispersions.
Unter anderem ist Acrylsäure durch heterogen katalysierte Gasphasen-Partialoxidation von Propan oder Propen und/oder Acrolein mit molekularem Sauerstoff an im festen Aggregatzustand befindlichen Katalysatoren bei erhöhter Temperatur erhältlich. Dabei werden die genannten Ausgangsgase, in der Regel mit inerten Gasen wie z.B. Stickstoff, C0 und/oder Wasserdampf verdünnt, im Gemisch mit molekularem Sauerstoff bei erhöhten Temperaturen sowie gegebenenfalls erhöhtem Druck über übergangsmetallische Mischoxidkatalysatoren geleitet und oxidativ in ein Acrylsäure enthaltendes Produktgasgemisch umgewandelt.Among other things, acrylic acid is obtainable by heterogeneously catalyzed gas phase partial oxidation of propane or propene and / or acrolein with molecular oxygen on catalysts in the solid state at elevated temperature. The starting gases mentioned, usually with inert gases such as Diluted nitrogen, CO and / or water vapor, passed in a mixture with molecular oxygen at elevated temperatures and, if appropriate, increased pressure over transition-metal mixed oxide catalysts and converted oxidatively into a product gas mixture containing acrylic acid.
Durch Aufnahme in ein geeignetes Absorptionsmittel, z.B. Wasser oder ein Gemisch aus 70 bis 75 Gew. -% Diphenylether und 25 bis 30 Gew. -% Diphenyl, kann eine Grundabtrennung der Acrylsäure aus dem Produktgasstrom erzielt werden (vgl. z.B. EP-A 297445 und DE-PS 2136396) . Durch Entfernung des Absorptionsmittels über destillative Trennverfahren wird normalerweise eine Acrylsäure erhalten, die noch kein reines Produkt ist, weshalb sie normalerweise als rohe Acrylsäure oder auch Rohacrylsäure bezeichnet wird, die auf un- terschiedliche Art und Weise weitergereinigt werden kann.A basic separation of acrylic acid from the product gas stream can be achieved by absorption in a suitable absorption medium, e.g. water or a mixture of 70 to 75% by weight diphenyl ether and 25 to 30% by weight diphenyl (cf. e.g. EP-A 297445 and DE-PS 2136396). Removal of the absorbent by means of distillative separation processes normally gives an acrylic acid which is not yet a pure product, which is why it is normally referred to as crude acrylic acid or also crude acrylic acid, which can be purified further in various ways.
Nachteilig an den vorgenannten destillativen Trennverfahren ist jedoch, daß bei ihnen trotz einer Mitverwendung von radikalischen Polymerisationsinhibitoren in einem solchen Umfang die Bildung von in unerwünschter Weise radikalisch polymerisierter Acrylsäure einhergeht, daß die Destillationsvorrichtungen in nahezu periodischen Abständen (in der Regel einige Wochen) zum Zweck der Polymerisatentfernung abgeschaltet werden müssen.A disadvantage of the above-mentioned distillation separation processes, however, is that in spite of the use of radical polymerization inhibitors, the formation of acrylic acid which has been radically polymerized in an undesirable manner goes hand in hand with the fact that the distillation devices are used at almost periodic intervals (usually a few weeks) for the purpose of Polymer removal must be switched off.
Aus der DE-A 19740253, der DE-A 19740252, den DE-A's 19814421, 19814387 und 19814375 ist ein wie eingangs beschriebenes Verfahren zur Herstellung von Acrylsäure bekannt, bei dem die Grundabtrennung der Rohacrylsäure durch fraktionierte Kondensation des Produktgasgemisches der heterogen katalysierten Gasphasen- Partia- loxidation vorgenommen wird. Eine solche Verfahrensweise ist auch Gegenstand der DE-A 19909923.DE-A 19740253, DE-A 19740252, DE-A's 19814421, 19814387 and 19814375 disclose a process for the preparation of acrylic acid as described in the introduction, in which the basic separation of the crude acrylic acid by fractional condensation of the product gas mixture of the heterogeneously catalyzed gas phase Partial oxidation is carried out. Such a procedure is also the subject of DE-A 19909923.
Als ein wesentlicher Vorteil der vorgenannten Grundabtrennung von Rohacrylsäure durch fraktionierte Kondensation hat es sich nun in überraschender und nicht verständlicher Weise herausgestellt, daß sie bei Mitverwendung von radikalischen Polymerisationsinhibitoren nur noch in deutlich geminderter Form von der Ausbildung radikalisch polymerisierter Acrylsäure gestört wird.A major advantage of the above-mentioned basic separation of crude acrylic acid by fractional condensation has now been found, in a surprising and incomprehensible manner, that when free-radical polymerization inhibitors are used, it is only disturbed in a significantly reduced form by the formation of radically polymerized acrylic acid.
Nachteilig ist jedoch nach wie vor, daß in kondensierter Phase befindliche Acrylsäure durch Michael -Addition an sich selbst sowie an das sich dabei bildende Acrylsäure-Dimere Acrylsäure-Oli- gomere (Michael -Addukte) bildet (der Begriff Acrylsäure-Oligomere meint in dieser Schrift stets die entsprechenden Michael -Addukte und nicht durch radikalische Polymerisation entstehende Acrylsäu- reoligomere) . Ein Beisein von Wasser, dem unvermeidbaren Nebenprodukt einer gasphasenkatalytisch oxidativen Herstellung von Acrylsäure, sowie erhöhte Temperaturen fördern die Bildung von Acrylsäure-01igomeren.However, it is still a disadvantage that acrylic acid in the condensed phase forms Michael addition to itself and to the acrylic acid dimer which forms in the process, acrylic acid oligomers (Michael adducts) (the term acrylic acid oligomers means in this document always the corresponding Michael adducts and acrylic acid oligomers not formed by radical polymerization). The presence of water, the unavoidable by-product of a gas-phase catalytically oxidative production of acrylic acid, and elevated temperatures promote the formation of acrylic acid oligomers.
Da Acrylsäure-Oligomere eine höhere Siedetemperatur als Acrylsäure aufweisen reichern sie sich sowohl im Rahmen einer destil- • lativen Abtrennung von Acrylsäure als auch bei einer fraktionierten Kondensation des Produktgasgemisches einer gasphasenkataly- tisch oxidativen AcrylSäureerzeugung in der Sumpfflüssigkeit an. Prinzipiell ist eine Bildung von Acrylsäure-Oligomeren unerwünscht, da sie die Ausbeute an Wertprodukt mindert.Since acrylic acid oligomers have a higher boiling temperature than acrylic acid, they accumulate in the bottom liquid both in the course of a distillative • separation of acrylic acid and in the case of fractional condensation of the product gas mixture of a gas-phase catalytically oxidative acrylic acid production. In principle, the formation of acrylic acid oligomers is undesirable because it reduces the yield of valuable product.
Es ist nun allgemein bekannt, daß die Bildung von Acrylsäure -Oli - gomeren ein reversibler Prozeß ist, der durch die Einwirkung erhöhter Temperaturen umgekehrt werden kann (vgl. z.B. DE-A 4101879, DE-A 19701737, DE-A 19536191, US-A 5734075, DE-A 2901783, US-A 3086046, US-A 2806878, DE-PS 1618129, Ullmanns Encyclopädie der technischen Chemie, 4. Auflage, Band 7, Verlag Chemie, Weinheim, S. 83, rechte Spalte sowie Ullmann's Encyclope- dia of Industrial Chemistry, Fifth, Completely Revised Edition, Verlag Chemie, Weinheim, Vol. AI, S. 167, rechte Spalte).It is now generally known that the formation of acrylic acid oligomers is a reversible process which can be reversed by the action of elevated temperatures (cf., for example, DE-A 4101879, DE-A 19701737, DE-A 19536191, US Pat. A 5734075, DE-A 2901783, US-A 3086046, US-A 2806878, DE-PS 1618129, Ullmann's Encyclopedia of Industrial Chemistry, 4th edition, Volume 7, Verlag Chemie, Weinheim, p. 83, right column and Ullmann's Encyclope - dia of Industrial Chemistry, Fifth, Completely Revised Edition, Verlag Chemie, Weinheim, Vol. AI, p. 167, right column).
Diese thermische Rückspaltung kann sowohl sauer als auch basisch katalysiert oder auch in Abwesenheit von Katalysatoren durchgeführt werden (vgl. z.B. EP-A 717031).This thermal cleavage can be catalyzed both acidic and basic or carried out in the absence of catalysts (see e.g. EP-A 717031).
Die Aufgabe der vorliegenden Erfindung bestand nun darin, in das wie eingangs beschriebene Verfahren zur Herstellung von Acryl- säure, bei dem die Grundabtrennung der Rohacrylsäure durch fraktionierte Kondensation des Produktgasgemisches der heterogen katalysierten Gasphasen-Partialoxidation durchgeführt wird, eine Rückspaltung der im Auslaß der Quenchflüssigkeit 1 enthaltenen Acrylsäure-Oligomeren so zu integrieren, daß die Laufzeit des Verfahrens, insbesondere jene der Kolonne zur fraktionierten Kondensation, im wesentlichen nicht gemindert wird.The object of the present invention was to split back the process in the process for the production of acrylic acid as described in the introduction, in which the basic separation of the crude acrylic acid is carried out by fractional condensation of the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation contained acrylic acid oligomers so that the running time of the process, in particular that of the column for fractional condensation, is essentially not reduced.
Demgemäß wurde ein Verfahren zur Herstellung von Acrylsäure, bei dem man durch heterogen katalysierte Gasphasen-Partialoxidation von Propan oder Propen und/oder Acrolein mit molekularem Sauerstoff an im festen Aggregatzustand befindlichen Katalysatoren bei erhöhter Temperatur ein Acrylsäure enthaltendes heißes Produkt - gasgemisch erzeugt, die Temperatur des heißen, Acrylsäure enthaltenden Produktgasgemisches durch direkte Kühlung mit einer Quenchflüssigkeit 1 verringert, den dabei nicht verdampfenden Teil der Quenchflüssigkeit 1, gegebenenfalls über einen indirekt kühlenden Wärmetauscher, im Kreis führt und als Auslaß einen Teil der nicht verdampften Quenchflüssigkeit 1 aus diesem Kreislauf ausschleust, anschließend das abgekühlte Produktgasgemisch in eine mit trennwirksamen Einbauten ausgerüstete Kolonne leitet, innerhalb der Kolonne in sich selbst aufsteigen läßt und dabei fraktioniert kondensiert, wobei im Seitenabzug eine rohe Acrylsäure und über den Kolonnensumpf und/oder über einen in Sumpfnähe gelegenen Seitenabzug Acrylsäure-Oligomere enthaltende Sumpfflüs- sigkeit und/oder Schwersiederfraktion (schwerer flüchtig alsAccordingly, a process for the preparation of acrylic acid was produced in which a hot product-gas mixture containing acrylic acid is produced at elevated temperature by heterogeneously catalyzed gas-phase partial oxidation of propane or propene and / or acrolein with molecular oxygen on catalysts in the solid state of aggregation, the temperature of hot, acrylic acid-containing product gas mixture reduced by direct cooling with a quench liquid 1, the non-evaporating part of the quench liquid 1, optionally via an indirectly cooling heat exchanger, is circulated and a part of the non-evaporated quench liquid 1 is discharged from this circuit as an outlet, then the cooled product gas mixture leads into a column equipped with separating internals, can rise within itself in the column and condenses fractionally, with a crude acrylic acid in the side draw and over the column sump and / or sump liquid and / or high boiler fraction (less volatile than
Acrylsäure) entnommen und als Quenchflüssigkeit 1 verwendet wird, gefunden, das dadurch gekennzeichnet ist, daß man den Auslaß der Quenchflüssigkeit 1 einem Spaltgefäß zuführt und in selbigem die im Auslaß der Quenchflüssigkeit 1 enthaltenen Acrylsäure-Oligomeren bei erhöhter Temperatur in Acrylsäure rückspaltet, dabei gasförmig aus der Flüssigphase entweichende Acrylsäure kondensiert 5 und das resultierende Kondensat in den Kreislauf der Quenchflüssigkeit 1 einspeist.Acrylic acid) is removed and used as quench liquid 1, which is characterized in that the outlet of the Feeds quench liquid 1 to a cracking vessel and in the same the acrylic acid oligomers contained in the outlet of quench liquid 1 are split back into acrylic acid at elevated temperature, acrylic acid escaping gaseously from the liquid phase is condensed 5 and the resulting condensate is fed into the circuit of quench liquid 1.
Die Einleitstelle des gequenchten Produktgasgemisches der kataly- tischen Gasphasenoxidation in die mit trennwirksamen EinbautenThe point of introduction of the quenched product gas mixture of the catalytic gas phase oxidation into those with separating internals
10 ausgerüstete Kolonne befindet sich in zweckmäßiger Weise unterhalb der trennwirksamen Einbauten. Prinzipiell kann die fraktionierte Kondensation innerhalb der Kolonne in an sich bekannter Weise durch indirekte Kühlung und/oder Erwärmung bewirkt werden. Es ist jedoch zweckmäßiger die fraktionierte Kondensation wie10 equipped column is conveniently located below the separating internals. In principle, the fractional condensation within the column can be effected in a manner known per se by indirect cooling and / or heating. However, it is more convenient to use fractional condensation
15 folgt zu bewirken. Über einen oberhalb der Einleitstelle und unterhalb der Einbauten angebrachten ersten Fangboden wird ein Teil der beim Aufsteigen des gequenchten Produktgasgemisches sich bildenden und/oder kondensierenden, höher als Acrylsäure siedenden, Substanzen entnommen. Ein Teil der entnommenen Schwersiederfrak-15 follows to effect. A part of the substances which form and / or condense and boil more than acrylic acid when the quenched product gas mixture rises is removed via a first collecting tray attached above the inlet point and below the internals. Part of the extracted high boiler fraction
20 tion wird im Gemisch mit der der Kolonne entnommenen Sumpfflüs- sigkeit als Quenchflüssigkeit 1 verwendet und die verbleibende Teilmenge der entnommenen Schwersiederfraktion wird in einem indirekten Wärmetauscher abgekühlt oder erwärmt und oberhalb des ersten, aber unterhalb eines in der unteren Kolonnenhälfte ange-20 tion is used in a mixture with the bottom liquid removed from the column as quench liquid 1 and the remaining part of the removed high boiler fraction is cooled or heated in an indirect heat exchanger and heated above the first but below one in the lower half of the column.
25 brachten zweiten Fangbodens in die Kolonne rückgeführt. Über den zweiten Fangboden wird im Seitenabzug als Mittelsiederfraktion die Rohacrylsäure entnommen, die im Normalfall eine Reinheit > 95 Gew.-% aufweist. Zweckmäßigerweise wird man die Rohacrylsäure weiteren destillativen und/oder kristallisativen Reinigungsstufen25 brought the second tray back into the column. The crude acrylic acid, which normally has a purity> 95% by weight, is taken off in the side draw as a medium boiler fraction via the second collecting tray. The crude acrylic acid is expediently used for further distillative and / or crystallizing purification stages
30 zuführen und wenigstens einen Teil der im Rahmen der Destillationen und/oder Kristallisationen anfallenden Sumpfflüssigkeiten und/oder Mutterlaugen unterhalb des zweiten, aber oberhalb des ersten Fangbodens in die Kolonne rückführen. Alternativ dazu kann die Rückführung der Sumpfflüssigkeiten und/oder Mutterlaugen auch30 supply and return at least a portion of the bottoms and / or mother liquors resulting from the distillations and / or crystallizations below the second but above the first tray into the column. Alternatively, the return of the bottom liquids and / or mother liquors can also
35 so erfolgen, daß sie in zwei Teilströme aufgeteilt werden, von denen einer unterhalb des zweiten, aber oberhalb des ersten Fang- bodens und der andere oberhalb des zweiten Fangbodens in die Kolonne rückgeführt wird. Der letztgenannte der beiden Teilströme wird in der Regel, bezogen auf die Gesamtrückführung, bis zu35 take place in such a way that they are divided into two partial streams, one of which is returned to the column below the second but above the first tray and the other above the second tray. The latter of the two partial flows is usually, based on the total return, up to
40 35 Gew. -% betragen. Aus dem am Kopf der Kolonne entweichenden40 35% by weight. From the one escaping at the top of the column
Leichtsiedergasstrom wird man zweckmäßig durch direkte Kühlung in einem an Einbauten freien oder Einbauten enthaltenden Raum mittels einer zweiten Quenchflüssigkeit (in dieser Schrift zum Zweck der Differenzierung als Quenchflüssigkeit 2 bezeichnet) im we-Low-boiling gas flow is expediently obtained by direct cooling in a space free of or containing internals by means of a second quench liquid (referred to in this document for the purpose of differentiation as quench liquid 2)
45 sentlichen Wasser sowie schwerer als Wasser flüchtige Bestandteile auskondensieren. Das dabei gewonnene Kondensat wird als Sauerwasser bezeichnet. Einen Teil des Sauerwassers wird man in sinnvoller Weise zur Erhöhung der Trennleistung am Kopf der Kolonne in selbige rückführen. Ein weiterer Teil des Sauerwassers wird zweckmäßigerweise ausgeschleust und entsorgt (z.B. verbrannt) und der verbleibende Teil des Sauerwassers wird üblicher - weise in einem externen Wärmetauscher indirekt abgekühlt und als Quenchflüssigkeit 2 verwendet.Condensate 45 substantial water and components that are more volatile than water. The condensate obtained is referred to as sour water. Part of the acid water is in sensible way to increase the separation performance at the top of the column in the same. A further part of the acid water is expediently discharged and disposed of (for example burned) and the remaining part of the acid water is usually cooled indirectly in an external heat exchanger and used as quench liquid 2.
Leichter als Wasser flüchtige Bestandteile des Leichtsiederstroms werden im wesentlichen gasförmig abgezogen und gegebenenfalls als Verdünnungsgas in die Gasphasenoxidation rückgeführt.Components of the low-boiling stream which are more volatile than water are drawn off essentially in gaseous form and, if appropriate, are returned to the gas phase oxidation as a diluent gas.
Alternativ kann man den Sauerwasserquench in die Kolonne für die fraktionierte Kondensation integrieren. In diesem Fall wird über einen weiteren Fangboden im oberen Teil der Kolonne wäßrige Rück- laufflüssigkeit entnommen, in einem Wärmetauscher indirekt gekühlt und bis auf den zu entsorgenden Auslaßanteil teilweise am Kopf der Kolonne und teilweise unterhalb des Fangbodens rückgeführt. Das gegebenenfalls in die Gasphasenoxidation rückzuführende Abgas verläßt die Kolonne in diesem Fall an deren Kopf.Alternatively, the acid water quench can be integrated into the column for the fractional condensation. In this case, aqueous reflux liquid is withdrawn via a further collecting tray in the upper part of the column, indirectly cooled in a heat exchanger and partly returned to the top of the column and partly below the collecting tray except for the outlet portion to be disposed of. In this case, the waste gas which is possibly to be returned to the gas phase oxidation leaves the column at the top thereof.
Der Quench des Produktgasgemisches der heterogen katalysierten Gasphasen-Partialoxidation mit der Quenchflüssigkeit 1 wird in der Regel in einem an Einbauten freien Raum ausgeführt. Die dabei Kühlgrenztemperatur erreichende Quenchflüssigkeit 1 wird, gegebe- nenfalls über einen externen Wärmetauscher, im Kreis geführt und zur Direktkühlung wiederverwendet. Ein Teil der Quenchflüssigkeit 1 wird aus diesem Kreislauf als Auslaß ausgeschleust und der erfindungsgemäß zu integrierenden Rückspaltung zugeführt.The quenching of the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation with the quenching liquid 1 is generally carried out in a free space in internals. The quench liquid 1 which reaches the cooling limit temperature is circulated, if necessary via an external heat exchanger, and reused for direct cooling. Part of the quench liquid 1 is discharged from this circuit as an outlet and fed to the cleavage to be integrated according to the invention.
Der Reinheitsgrad der über Seitenabzug aus der fraktionierten Kondensationskolonne entnommenen Rohacrylsäure beträgt üblicherweise > 95 Gew.-%. D.h., die Rohacrylsäure enthält zu wenigstens 95 % ihres Gewichtes Acrylsäuremoleküle.The degree of purity of the crude acrylic acid withdrawn from the fractionated condensation column via side draw is usually> 95% by weight. That is, the crude acrylic acid contains at least 95% of its weight acrylic acid molecules.
Die Polymerisationsinhibierung des erfindungsgemäßen Verfahrens kann im wesentlichen wie in der DE-A 19909923 beschrieben durchgeführt werden. D.h., sie kann beispielsweise dadurch erfolgen, daß am Kopf der Kondensationskolonne Phenothiazin oder ein Gemisch aus einem N-Oxyl-Radikal und einer Benzolverbindung, die zwei über Heteroatome an den aromatischen Ring gebundene Substi- tuenten und wenigstens einen beweglichen Wasserstoff aufweist, zugesetzt wird. Eine günstigere Form der Polymerisationsinhibierung ist dadurch gekennzeichnet, daß sie ausschließlich mittels N-Oxyl -Radikalen (z.B. die in der EP-A 765856 genannten) erfolgt. Das sind Verbindungen, die wenigstens eine Gruppe -N-O« aufweisen.The polymerization inhibition of the process according to the invention can be carried out essentially as described in DE-A 19909923. This means that it can be carried out, for example, by adding phenothiazine or a mixture of an N-oxyl radical and a benzene compound which has two substituents bonded to the aromatic ring via heteroatoms and at least one mobile hydrogen at the top of the condensation column. A more favorable form of polymerization inhibition is characterized in that it takes place exclusively by means of N-oxyl radicals (for example those mentioned in EP-A 765856). These are compounds that have at least one group -NO «.
Erfindungsgemäß bevorzugte N-Oxyl -Radikale sind die Pyrroli- din- 1- oxyl -Typen und die Piperidin-1 -oxyl -Typen. Beispielhaft genannt seien 4, 4' , 4"-Tris- (2 , 2, 6, 6 - tetramethylpiperi- din-l-oxyl)phosphit, 2,2, 6, 6 -Tetramethylpiperidin- 1-oxyl (TEMPO) , 4 -Hydroxy- 2, 2, 6, 6- tetramethylpiperidin-1 -oxyl (4-OH-TEMPO) ,According to the invention, preferred N-oxyl radicals are the pyrrolidine-1-oxyl types and the piperidine-1 -oxy types. Examples include 4, 4 ', 4 "-Tris- (2, 2, 6, 6 - tetramethylpiperidine-l-oxyl) phosphite, 2,2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPO), 4 -Hydroxy- 2, 2, 6, 6-tetramethylpiperidine-1 -oxyl (4-OH-TEMPO),
4-Oxo-2, 2,6, 6- tetramethylpiperidin- 1 -oxyl (4 -Oxo -TEMPO) , 4 -Dirne - thylamino- 2,2,6,6- tetramethylpiperidin-1 -oxyl,4-oxo-2, 2,6, 6-tetramethylpiperidine-1 -oxyl (4 -oxo -TEMPO), 4 -dirne - thylamino-2,2,6,6-tetramethylpiperidine-1 -oxyl,
4 -Amino-2, 2,6,6 - tetramethylpiperidin-1 -oxyl, 4-Ethanoyl- oxy-2, 2,6, 6- tetramethylpiperidin-1 -oxyl, 2,2,5, 5 -Tetramethylpyr- rolidin- 1-oxyl und 3 -Amino-2 , 2 , 5, 5 -tetramethylpyrrolidin-1 -oxyl . Die N-Oxyl -Inhibitoren werden vorzugsweise als 0,1 bis 2 gew. -%ige Lösungen in Wasser und/oder in Sauerwasser eingesetzt. Die Zugabe der wäßrigen N-Oxyl- Inhibitorlösung erfolgt zweckmäßigerweise im oberen Viertel der Kondensationskolonne und/ oder in den Sauerwasserquench. Die wäßrige Lösung kann nur ein N-Oxyl -Radikal oder ein Gemisch von N-Oxyl -Radikalen enthalten. Eine ausreichende Inhibierung kann bereits durch Zugabe einer wäßrigen Lösung erzielt werden, die ausschließlich 4-OH-TEMPO als Polymerisationsinhibitor enthält .4-amino-2, 2,6,6-tetramethylpiperidine-1 -oxyl, 4-ethanoyl-oxy-2, 2,6, 6-tetramethylpiperidine-1 -oxyl, 2,2,5, 5-tetramethylpyrrolidine 1-oxyl and 3-amino-2, 2, 5, 5 -tetramethylpyrrolidin-1 -oxyl. The N-oxyl inhibitors are preferably 0.1 to 2 wt. -% solutions used in water and / or in acid water. The aqueous N-oxyl inhibitor solution is expediently added in the upper quarter of the condensation column and / or in the acid water quench. The aqueous solution can contain only one N-oxyl radical or a mixture of N-oxyl radicals. Adequate inhibition can already be achieved by adding an aqueous solution which contains only 4-OH-TEMPO as a polymerization inhibitor.
Die Zugabemenge der einzusetzenden N-Oxyl -Inhibitoren wird in zweckmäßiger Weise so bemessen, daß die aus der Kolonne entnommene Schwersiederfraktion und Sumpfflüssigkeit 1 bis 1000 gew.ppm, bezogen auf das Gewicht der Schwersiederfraktion, an N-Oxyl -Inhibitoren enthält. Da die aus der Kolonne entnommene Schwersiederfraktion und Sumpfflüssigkeit als Quenchflüssigkeit 1 dient, wird das Quenchsystem 1 automatisch mitstabilisiert. Bei Bedarf kann das Quenchsystem 1 durch Zusatz einer Phenothiazin- verbindung co- stabilisiert werden. Als solche Phenothiazinverbin- düngen kommen beispielsweise Phenothiazin (PTZ) selbst,The addition amount of the N-oxyl inhibitors to be used is expediently such that the high boiler fraction and bottom liquid removed from the column contain 1 to 1000 ppm by weight, based on the weight of the high boiler fraction, of N-oxyl inhibitors. Since the high boiler fraction and bottom liquid removed from the column serve as quench liquid 1, the quench system 1 is automatically stabilized as well. If necessary, the quench system 1 can be stabilized by adding a phenothiazine compound. Examples of such phenothiazine compounds include phenothiazine (PTZ) itself,
Bis- (α-methylbenzyl)phenothiazin, 3, 7 -Dioctylphenothiazin und Bis- (α-dimethylbenzyl) phenothiazin in Betracht, unter denen das Phenothiazin bevorzugt ist. Letzteres gilt insbesondere dann, wenn 4-OH-TEMPO zur Stabilisierung der Kondensationskolonne mit- verwendet oder ausschließlich verwendet wird. Ein solcher Pheno- thiazinzusatz kann, bezogen auf das Gewicht der Quenchflüssigkeit, 1 bis 500 gew.ppm betragen. Zweckmäßigerweise erfolgt ein solcher Zusatz einer Phenothiazinverbindung in Acrylsäure, vorzugsweise in über Seitenabzug entnommener Rohacrylsäure gelöst (typisch 0,1 bis 2 gew. -%ig) . Der zur Polymerisationsinhibierung erforderliche Bedarf an N-Oxyl -Radikalen läßt sich dadurch absenken, daß die weniger anspruchsvolle Sauerwasserstabilisierung, d.h., die Stabilisierung des Quenchsystems 2, ersatzweise oder in Kombination mit einer wäßrigen Lösung wenigstens einer Benzolverbindung, die zwei über Heteroatome an den aromatischen Ring gebundene Substituenten und wenigstens einen beweglichen Wasserstoff aufweist (z.B. die in der EP-A 766856 genannten), z.B. einer Phenolverbindung, durchgeführt wird (typisch 0,1 bis 2 gew. -%ige Lösung). Als solche Phe- nolverbindungen kommen beispielsweise Hydrochinon oder Methochi- non (p-Methoxyphenol = MEHQ) in Betracht, unter denen letztere bevorzugt ist. Dies gilt insbesondere dann, wenn im Kopfbereich der Kolonne ausschließlich 4-OH-TEMPO eingesetzt und das Quenchsystem mit PTZ co- stabilisiert wird. In der Regel werden, bezogen auf das Gewicht des Sauerwassers, 1 bis 500 gew.ppm wenigstens einer PhenolVerbindung für eine solche Sauerwasserinhibierung verwendet.Bis- (α-methylbenzyl) phenothiazine, 3, 7 -dioctylphenothiazine and bis- (α-dimethylbenzyl) phenothiazine, among which phenothiazine is preferred. The latter applies in particular when 4-OH-TEMPO is used for the stabilization of the condensation column or is used exclusively. Such an addition of phenothiazine, based on the weight of the quench liquid, can be 1 to 500 ppm by weight. Such a phenothiazine compound is expediently added in acrylic acid, preferably dissolved in crude acrylic acid taken off via a side draw (typically 0.1 to 2% by weight). The need for N-oxyl radicals required to inhibit polymerization can be reduced in that the less demanding stabilization of acidic water, ie the stabilization of the quench system 2, alternatively or in combination with an aqueous solution of at least one benzene compound, the two via heteroatoms on the aromatic ring has bound substituents and at least one mobile hydrogen (for example those mentioned in EP-A 766856), for example a phenol compound, is carried out (typically 0.1 to 2% by weight solution). Examples of such phenol compounds are hydroquinone or methoquinone (p-methoxyphenol = MEHQ), of which the latter is preferred. This applies in particular if only 4-OH-TEMPO is used in the top area of the column and the quench system is stabilized with PTZ. As a rule, 1 to 500 ppm by weight, based on the weight of the acid water, of at least one phenol compound are used for such acid water inhibition.
Eine alternative Polymerisationsinhibierung besteht darin, am Kopf der Kondensationskolonne eine wäßrige Lösung von MEHQ aufzugeben und die Sauerwasserinhibierung ebenfalls durch Zusatz einer Lösung von MEHQ in Wasser und/oder in Sauerwasser durchzuführen. Zusätzlich wird im Mittelteil der Kondensationskolonne sowie gegebenenfalls im Produktgasgemischquench (Quench 1) eine Lösung von PTZ in Acrylsäure (z.B. Rohacrylsäure) aufgegeben.An alternative polymerization inhibition is to add an aqueous solution of MEHQ to the top of the condensation column and to carry out the acid water inhibition likewise by adding a solution of MEHQ in water and / or in acid water. In addition, a solution of PTZ in acrylic acid (e.g. crude acrylic acid) is introduced in the middle section of the condensation column and, if appropriate, in the product gas mixture quench (Quench 1).
Wie bereits erwähnt, wird das Quenchsystem 1 über die Stabilisierung der Kondensationskolonne automatisch mitstabilisiert und kann bei Bedarf durch Zusatz von Phenothiazin und/oder Metochinon co-stabilisiert werden.As already mentioned, the quench system 1 is automatically stabilized via the stabilization of the condensation column and can be co-stabilized if necessary by adding phenothiazine and / or metoquinone.
Die dabei erzielte Polymerisationsinhibierung der Quenchflüssigkeit 1 ist in der Regel auch ausreichend, um bei der erfindungs- gemäß zu integrierenden Rückspaltung eine ausreichende Stabilität des Auslaß der Quenchflüssigkeit 1 gegen unerwünschteThe polymerization inhibition of the quench liquid 1 achieved in this way is generally also sufficient to ensure sufficient stability of the outlet of the quench liquid 1 against undesired during the cleavage to be integrated according to the invention
Polymerisatbildung zu gewährleisten. Hingegen werden bei der Kondensation der im Rahmen der Rückspaltung gasförmig entweichenden, Acrylsäure enthaltenden, Dämpfe die Kondensatoroberflächen zweckmäßigerweise extra inhibiert (die in der Quenchflüssigkeit 1 enthaltenen Inhibitoren verdampfen in der Regel nicht mit) . Diese Polymerisationsinhibierung der Kondensatoroberflächen wird man mit Vorteil mit denselben Polymerisationsinhibitoren vornehmen, die auch zur Inhibierung der Kondensationskolonne verwendet und/ oder empfohlen wurden. Verwendet man zur Stabilisierung der Kondensationskolonne und des Sauerwasserquench z.B. ausschließlich 4 -OH- EMPO, so ist es sinnvoll, die Kondensatoroberflächen auch ausschließlich mit 4-OH- TEMPO, zweckmäßig in Rohacrylsäure gelöst, vorzunehmen. Selbst - verständlich können die Kondensatoroberflächen aber auch mittels PTZ, MEHQ und/oder Hydrochinon co- stabilisiert oder ausschließlich stabilisiert werden.To ensure polymer formation. On the other hand, the condenser surfaces are expediently additionally inhibited during the condensation of the vapors escaping in the course of the splitting-off, containing acrylic acid (the inhibitors contained in the quenching liquid 1 generally do not also evaporate). This polymerization inhibition of the condenser surfaces will advantageously be carried out using the same polymerization inhibitors which were also used and / or recommended for inhibiting the condensation column. If, for example, only 4 -OH-EMPO is used to stabilize the condensation column and the acid water quench, it makes sense to carry out the condenser surfaces exclusively with 4-OH-TEMPO, appropriately dissolved in crude acrylic acid. Of course, the capacitor surfaces can also be stabilized by means of PTZ, MEHQ and / or hydroquinone or only stabilized.
In der Regel wird die erfindungsgemäß zu integrierende Rück- Spaltung bei einem Druck von < 1 bar und bei einer Temperatur von 130 bis 250°C durchgeführt. Mit Vorteil beträgt der Druck für die Rückspaltung 25 bis 600, vorzugsweise 100 bis 300 mbar. Die Rück- spalttemperatur liegt zweckmäßigerweise bei 140 bis 230°C, vorzugsweise bei 160 bis 200°C. Wird die Rückspaltung kontinuier- lieh durchgeführt (das erfindungsgemäße Verfahren wird vorzugsweise kontinuierlich durchgeführt) , so sollte die Verweilzeit im Rückspaltreaktor etwa 0,5 bis 3 h betragen. In einfacher Weise läßt sich die erfindungsgemäß zu integrierende Rückspaltung in einem beheizbaren Rührreaktor durchführen. Wie in der US-A 5734075 sowie in der DE-A 4101879 beschrieben, läßt sich die Rückspaltung der im Auslaß des Quenchsystems 1 enthaltenen Acryl - säure-Oligomere ohne Zusatz spezieller saurer oder basischer Spaltkatalysatoren durchführen. Mit Vorteil wird man die Rückspaltung jedoch im Beisein von Spaltkatalysatoren durchführen. Als solche kommen z.B. Dodecylbenzolfulfonsäure, p-Toluolsulfon- säure, Schwefelsäure oder die festen sauren Katalysatoren der JP-A 3-178949 in Betracht.As a rule, the cleavage to be integrated according to the invention is carried out at a pressure of <1 bar and at a temperature of 130 to 250 ° C. The pressure for the cleavage is advantageously 25 to 600, preferably 100 to 300 mbar. The cleavage temperature is expediently from 140 to 230 ° C., preferably from 160 to 200 ° C. If the cleavage is carried out continuously (the process according to the invention is preferably carried out continuously), the residence time in the cleavage reactor should be about 0.5 to 3 h. The cleavage to be integrated according to the invention can be carried out in a simple manner in a heatable stirred reactor. As described in US-A 5734075 and in DE-A 4101879, the cleavage of the acrylic acid oligomers contained in the outlet of the quench system 1 can be carried out without the addition of special acidic or basic cleavage catalysts. However, the cleavage will advantageously be carried out in the presence of cleavage catalysts. As such, e.g. Dodecylbenzenesulfonic acid, p-toluenesulfonic acid, sulfuric acid or the solid acid catalysts of JP-A 3-178949.
Insbesondere im Fall einer Polymerisationsinhibierung mittels N- Oxyl -Radikalen, vor allem wenn 4-OH-TEMPO als alleinigerEspecially in the case of polymerization inhibition using N-oxyl radicals, especially when 4-OH-TEMPO is the only one
Polymerisationsinhibitor oder als co- Stabilisator des Quenchsystems 1 verwendet wird, ist es zweckmäßig die Rückspaltung durch Zusatz eines anorganischen Salzes, dessen Zusatz zu einer wäßrigen Lösung einer starken Brönsted- Säure den pH-Wert der wäßrigen Lösung ins alkalische verschiebt, vorzunehmen, wie es z.B. die DE-C 2407236 empfiehlt. Bezogen auf die der Rückspaltung zu unterwerfende Menge an Auslaß der Quenchflüssigkeit 1 wird die zuzusetzende Menge an basischem Rückspaltkatalysator in der Regel 0,1 bis 5 Gew. -% betragen. Beispiele für erfindungsgemäß geei- gnete Rückspaltkatalysatoren sind KOH, K2C03, KHC03, NaOH, Na2C03, NaHC03, LiOH, Li2C03 und CaC03.Polymerization inhibitor or used as co-stabilizer of the quench system 1, it is expedient to carry out the cleavage by adding an inorganic salt, the addition of which to an aqueous solution of a strong Bronsted acid shifts the pH of the aqueous solution to alkaline, as is the case, for example DE-C 2407236 recommends. Based on the amount of outlet of the quench liquid 1 to be subjected to the cleavage, the amount of basic cleavage catalyst to be added will generally be 0.1 to 5% by weight. Examples of cleavage catalysts suitable according to the invention are KOH, K 2 C0 3 , KHC0 3 , NaOH, Na 2 C0 3 , NaHC0 3 , LiOH, Li 2 C0 3 and CaC0 3 .
D.h., geeignete Rückspaltkatalysatoren sind insbesondere die Alkali- und/oder Erdalkalisalze von schwachen anorganischen oder organischen Brönstedsäuren wie z.B. Phosphorsäure, Borsäure, Kohlensäure, Ameisensäure oder Essigsäure. Mit anderen Worten eignen sich somit als Rückspaltkatalysatoren vor allem Alkali- und/oder Erdalkaliphosphate, -borate, -carbonate, -hydrogencarbonate, -formiate und -acetate.In other words, suitable cleavage catalysts are, in particular, the alkali and / or alkaline earth metal salts of weak inorganic or organic Bronsted acids such as, for example, phosphoric acid, boric acid, carbonic acid, formic acid or acetic acid. In other words, alkali and / or are particularly suitable as cleavage catalysts Alkaline earth metal phosphates, borates, carbonates, bicarbonates, formates and acetates.
Vorzugsweise wird man die Rückspaltkatalysatoren so wählen, daß sie unter den Rückspaltbedingungen im Auslaß der Quenchflüssigkeit 1 löslich sind. Gemäß US-A 4293347 wirkt sich auch ein Beisein von Dialkylphthalaten vorteilhaft auf die relevante Rückspaltung aus.The cleavage catalysts will preferably be selected so that they are soluble in the outlet of the quench liquid 1 under the cleavage conditions. According to US-A 4293347, the presence of dialkyl phthalates also has an advantageous effect on the relevant cleavage.
Der im Spaltreaktor verbleibende schwerflüchtige Rückstand wird beim erfindungsgemäßen Verfahren regelmäßig seiner Entsorgung, z.B. seiner Verbrennung, zugeführt. Dazu kann der Spaltrückstand gegebenenfalls mit hydrophilen organischen Flüssigkeiten wie organischen Säuren (z.B. Ethylhexansäure, Propionsäure) oder Rück- stände aus der Herstellung dieser Säuren aber auch mit Alkanolen wie Ethanol oder Methanol verdünnt werden.The hardly volatile residue remaining in the cleavage reactor is regularly disposed of in the process according to the invention, e.g. its combustion. For this purpose, the cleavage residue can optionally be diluted with hydrophilic organic liquids such as organic acids (e.g. ethylhexanoic acid, propionic acid) or residues from the production of these acids, but also with alkanols such as ethanol or methanol.
An dieser Stelle sei noch festgehalten, daß der im erfindungs- gemäßen Verfahren zu verwendenden Quenchflüssigkeit 1 bei Bedarf eine höher als Acrylsäure siedende inerte organische Flüssigkeit zugesetzt werden kann, die die Quenchflüssigkeit 1 fluid hält.At this point it should also be noted that the quench liquid 1 to be used in the process according to the invention can, if necessary, be added with an inert organic liquid which boils higher than acrylic acid and which keeps the quench liquid 1 fluid.
Als solche hochsiedenden inerten organischen Flüssigkeiten kommen insbesondere alle diejenigen in Betracht, die in der DE-A 2136396 und in der DE-A 4308087 empfohlen werden. Dies sind im wesentlichen Flüssigkeiten, deren Siedepunkt bei Normaldruck oberhalb von 160°C liegt. Beispielhaft genannt seien Ethylhexansäure, N-Methyl- pyrrolidon, Mittelölfraktionen aus der Paraffindestillation, Diphenylether, Diphenyl oder Mischungen der vorgenannten Flüssig - keiten wie z.B. ein Gemisch aus 70 bis 75 Gew.-% Diphenylether und 25 bis 30 Gew.-% Diphenyl. Günstig sind die Verwendung eines Gemisches bestehend aus einer Mischung aus 70 bis 75 Gew. -% Diphenylether und 25 bis 30 Gew. - Diphenyl, sowie, bezogen auf diese Mischung, 0,1 bis 25 Gew. -% o-Dimethylphthalat .Such high-boiling inert organic liquids are in particular all those which are recommended in DE-A 2136396 and in DE-A 4308087. These are essentially liquids whose boiling point at normal pressure is above 160 ° C. Examples include ethylhexanoic acid, N-methylpyrrolidone, middle oil fractions from paraffin distillation, diphenyl ether, diphenyl or mixtures of the aforementioned liquids, such as a mixture of 70 to 75 wt .-% diphenyl ether and 25 to 30 wt .-% diphenyl. It is favorable to use a mixture consisting of a mixture of 70 to 75% by weight of diphenyl ether and 25 to 30% by weight of diphenyl, and, based on this mixture, 0.1 to 25% by weight of o-dimethylphthalate.
Im vorgenannten Fall wird bei der Rückspaltung wenigstens eine Teilmenge der mitverwendeten inerten organischen Flüssigkeit mit- verdampfen. Verbleibt eine Teilmenge der organischen Flüssigkeit im Spaltrückstand, kann selbiger einer Aufarbeitung zugeführt werden, in der das mitverwendete Lösungsmittel, z.B. destillativ, abgetrennt und in den Quench 1 rückgeführt wird. Die verbleibenden Schwersieder werden entsorgt.In the aforementioned case, at least a part of the inert organic liquid which is also used is also evaporated during the cleavage. If a portion of the organic liquid remains in the cleavage residue, it can be subjected to workup in which the solvent used, e.g. distilled, separated and recycled to quench 1. The remaining high boilers are disposed of.
Häufig sind die heißen Produktgasgemische der heterogen kataly- sierten Gasphasenoxidation wie folgt zusammengesetzt:The hot product gas mixtures of the heterogeneously catalyzed gas phase oxidation are often composed as follows:
1 bis 30 Gew. -% Acrylsäure, 0,01 bis 3 Gew.-% Essigsäure, 0,01 bis 1 Gew.-% Propionsäure,1 to 30% by weight of acrylic acid, 0.01 to 3% by weight of acetic acid, 0.01 to 1% by weight of propionic acid,
0,01 bis 0,5 Gew.-% Maleinsäure/Maleinsäureanhydrid, 0,05 bis 1 Gew.- Acrolein, 0,05 bis 1 Gew.-% Formaldehyd, 0,1 bis 1 Gew. -% Furfural, 0,01 bis 0,5 Gew.-% Benzaldehyd, 0,01 bis 1 Gew.-% Propen, 0,05 bis 10 Gew.-% Sauerstoff, 1 bis 30 Gew. - Wasser und0.01 to 0.5% by weight of maleic acid / maleic anhydride, 0.05 to 1% by weight of acrolein, 0.05 to 1% by weight of formaldehyde, 0.1 to 1% by weight of furfural, 0.01 up to 0.5% by weight of benzaldehyde, 0.01 to 1% by weight of propene, 0.05 to 10% by weight of oxygen, 1 to 30% by weight of water and
als Restmenge inerte Gase wie z.B. Stickstoff, Kohlenoxide, Methan und Propan.as a residual amount of inert gases such as Nitrogen, carbon oxides, methane and propane.
Die Gasphasenoxidation selbst kann z.B. in zwei aufeinanderfolgenden Oxidationsstufen durchgeführt werden, wie sie in der EP-A 700 714 und in der EP-A 700 893 beschrieben sind. Selbstverständlich können aber auch die in der DE-A 19740253 sowie die in der DE-A 19740252 zitierten Gasphasenoxidationen zur Anwendung kommen.The gas phase oxidation itself can e.g. be carried out in two successive oxidation stages, as described in EP-A 700 714 and in EP-A 700 893. Of course, the gas phase oxidations cited in DE-A 19740253 and in DE-A 19740252 can also be used.
In der Regel beträgt die Temperatur des die Gasphasenoxidation verlassenden Produktgasgemisches 150 bis 350°C, meist 200 bis 300°C. Im Quenchsystem 1 wird das heiße Produktgasgemisch übli- cherweise auf eine Temperatur von 100 bis 180°C abgekühlt, mit der es dann in den untersten Abschnitt der Trennkolonne geführt wird. Der in der Kolonne herrschende Betriebsdruck beträgt in der Regel 0,5 bis 5 bar, häufig 0,5 bis 3 bar und vielfach 0,5 bis 2 bar.As a rule, the temperature of the product gas mixture leaving the gas phase oxidation is 150 to 350 ° C., usually 200 to 300 ° C. In the quench system 1, the hot product gas mixture is usually cooled to a temperature of 100 to 180 ° C., at which it is then led into the lowest section of the separation column. The operating pressure in the column is generally 0.5 to 5 bar, often 0.5 to 3 bar and often 0.5 to 2 bar.
Als Quenchvorrichtung 1 können alle im Stand der Technik für diesen Zweck bekannten Vorrichtungen (z.B. Sprühwäscher, Venturiwä- scher, Blasensäulen oder sonstige Apparate mit berieselten Oberflächen) eingesetzt werden, wobei erfindungsgemäß vorzugsweise Venturi -Wäscher oder Sprühkühler verwendet werden.All devices known in the prior art for this purpose (e.g. spray washers, venturi washers, bubble columns or other apparatuses with sprinkled surfaces) can be used as the quenching device 1, with venturi washers or spray coolers preferably being used according to the invention.
Zur indirekten Kühlung oder Erwärmung der Quenchflüssigkeit 1 eignen sich alle gängigen Wärmeüberträger oder Wärmetauscher. Als bevorzugt seien Rohrbündelwärmetauscher, Plattenwärmetauscher und Luftkühler genannt. Die Temperatur der Quenchflüssigkeit 1 be- trägt nach Verlassen des Wärmetauschers normalerweise 70 bisAll common heat exchangers or heat exchangers are suitable for indirect cooling or heating of the quench liquid 1. Pipe bundle heat exchangers, plate heat exchangers and air coolers may be mentioned as preferred. The temperature of the quench liquid 1 is normally 70 to after leaving the heat exchanger
200°C, häufig 100 bis 150°C. Geeignete Kühlmedien sind Luft beim entsprechenden Luftkühler und Kühlflüssigkeiten, insbesondere Wasser, bei den anderen Kühlvorrichtungen. Als Kolonneneinbauten kommen prinzipiell alle gängigen Einbauten in Betracht, insbeson- dere Böden, Packungen und/oder Füllkörper. Von den Böden sind200 ° C, often 100 to 150 ° C. Suitable cooling media are air for the corresponding air cooler and cooling liquids, in particular water, for the other cooling devices. In principle, all common internals come into consideration as column internals, in particular trays, packings and / or packing elements. Of the floors are
Glockenböden, Siebböden, Ventilböden und/oder Dual -Flow-Böden bevorzugt. In typischer Weise beträgt bei einer Bodenkolonne die Gesamtzahl an Trennbόden 20 bis 80 (für den Fall, daß der Sauer- wasserquench in die Kolonne integriert ist bis 100) , bevorzugt 50 bis 80 (bzw. 100). Die im Seitenabzug entnommene Rohacrylsäure enthält in der Regel nochBell bottoms, sieve bottoms, valve bottoms and / or dual flow bottoms preferred. In the case of a tray column, the Total number of separating plates 20 to 80 (in the event that the acid water quench is integrated in the column up to 100), preferably 50 to 80 (or 100). The crude acrylic acid removed in the side hood usually still contains
0,1 bis 2 Gew.-% Essigsäure, 0,5 bis 5 Gew.-% Wasser,0.1 to 2% by weight of acetic acid, 0.5 to 5% by weight of water,
0,05 bis 1 Gew. -% niedermolekulare Aldehyde, 0,001 bis 1 Gew.-% Maleinsäure und/oder deren Anhydrid sowie 1 bis 500 Gew.ppm Polymerisationsinhibitor,0.05 to 1% by weight of low molecular weight aldehydes, 0.001 to 1% by weight of maleic acid and / or its anhydride and 1 to 500% by weight of polymerization inhibitor,
jeweils bezogen auf das Gewicht der Rohacrylsäure. Die Temperatur im Sumpf der Kolonne liegt in typischer Weise bei 90 bis 130°C, wohingegen die Kopftemperatur normalerweise 40 bis 80, häufig 50 bis 70°C beträgt. Die Entnahmetemperatur der Rohacrylsäure liegt meist bei 80 bis 110°C. Die Rückführtemperatur der Schwersieder beträgt beim Eintritt in die Kolonne in typischer Weise 95 bis 115°C. Die Rückführtemperatur des Sauerwassers in die Kolonne liegt in der Regel bei 25 bis 35°C. Prinzipiell kann das Quench- System 2 wie das Quenchsystem 1 gestaltet werden. Selbstverständlich kann erfindungsgemäß die Entnahme der Rohacrylsäure auch über mehrere in kurzen Abständen aufeinanderfolgende Fangböden vorgenommen werden.each based on the weight of the crude acrylic acid. The temperature in the bottom of the column is typically 90 to 130 ° C, whereas the top temperature is normally 40 to 80, often 50 to 70 ° C. The extraction temperature of the crude acrylic acid is usually 80 to 110 ° C. The return temperature of the high boilers when entering the column is typically 95 to 115 ° C. The return temperature of the acid water to the column is usually 25 to 35 ° C. In principle, the quench system 2 can be designed like the quench system 1. Of course, according to the invention, the crude acrylic acid can also be removed via a plurality of collecting trays which follow one another at short intervals.
Die als Mittelsiederfraktion entnommene Rohacrylsäure kann erfindungsgemäß zum Zwecke der Weiterreinigung z.B. einer Kristallisation zugeführt werden. Hierbei wird in der Regel ohne Zusatz eines Lösungsmittels, insbesondere ohne Zusatz eines organischen Lösungsmittels gearbeitet. Das zu verwendende Kristallisations- verfahren unterliegt keiner Beschränkung. Die Kristallisation kann kontinuierlich oder diskontinuierlich, einstufig oder mehrstufig bis zu nahezu beliebigen Reinheitsgraden durchgeführt werden. Bei Bedarf kann der kristallisativ zu reinigenden Rohacrylsäure vorab einer Kristallisation Wasser zugesetzt werden (bezogen auf die enthaltene Menge an Acrylsäure bis zu 10 Gew. -% oder mehr, vorzugsweise bis zu 5 Gew. -%) . Ein solcher Zusatz erleichtert die Abtrennung von in der Rohacrylsäure als Nebenprodukt enthaltener Essigsäure, da diese im Beisein von Wasser in geringerem Ausmaß in die Acrylsäurekristalle eingebaut wird. Au- ßerdem mindert ein Beisein von Wasser die Verkrustungsneigung.According to the invention, the crude acrylic acid removed as a medium boiler fraction can be used for further purification, e.g. be supplied to a crystallization. This is usually carried out without the addition of a solvent, in particular without the addition of an organic solvent. The crystallization process to be used is not restricted. The crystallization can be carried out continuously or discontinuously, in one or more stages up to almost any degree of purity. If necessary, water can be added to the crude acrylic acid to be crystallisatively purified beforehand (based on the amount of acrylic acid up to 10% by weight or more, preferably up to 5% by weight). Such an addition facilitates the removal of acetic acid contained as a by-product in the crude acrylic acid, since this is incorporated to a lesser extent into the acrylic acid crystals in the presence of water. In addition, the presence of water reduces the tendency to crust.
Es überrascht, daß eine veresterungsgerechte Rohacrylsäure bereits durch eine einzige Kristallisationsstufe erzielt werden kann. Zweckmäßigerweise wird diese Kristallisationsstufe als Sus- pensionskristallisation ausgeführt. Als Kristaller wird dazu vorteilhaft ein Trog verwendet, in welchem gewischte Kühlplatten (die innerlich von einem Kühlmedium durchflössen werden) hinter- einander hängend angeordent sind. Durch das Wischen der Kühlplatten wird die Ausbildung einer Schichtkristallisation unterdrückt. Die Rohacrylsäure wird von hinten nach vorne kontinuierlich durch den Kristaller geführt (gepumpt oder Überlaufgeregelt) . Die ein- phasige Rohacrylsäure verdickt dabei zu einer zweiphasigen,It is surprising that crude acrylic acid suitable for esterification can be achieved with a single crystallization step. This crystallization stage is expediently carried out as a suspension crystallization. A trough in which wiped cooling plates (through which a cooling medium flows internally) is advantageously used as the crystallizer. are arranged hanging from each other. Wiping the cooling plates suppresses the formation of layer crystallization. The crude acrylic acid is continuously fed through the crystallizer from the back to the front (pumped or overflow controlled). The single-phase crude acrylic acid thickened to a two-phase,
Acrylsäurekristalle als feste Phase enthaltenden, Suspension. Ein diesbezüglich besonders geeigneter Kristaller ist ein Kristaller der Fa. GMF Gouda (Holland) vom Typ Kühlscheibenkristallisator (Cooling Disc Crystallizer) . In zweckmäßiger Weise werden die ge- bildeten Kristalle aus der vorgenannten Suspension mittels einer Zentrifuge abgetrennt (z.B. eine solche der Fa. Siebtechnik, vom Typ Schubzentrifuge SHS mit konischer Siebtrommel) und bei Bedarf mit bereits weiter gereinigter Rohacrylsäure gewaschen und/oder einem später beschriebenen Schwitzen unterworfen. Das Abtrennen und Waschen der Suspensionskristalle kann aber auch vorteilhaft in einer Waschkolonne durchgeführt werden, wie es z.B. in der EP-A 97405, US-A 3872009, EP-A 98637, EP-A 305316, EP-A 105524 und der WO 84/00118 beschrieben ist. Dann werden die Kristalle üblicherweise in einen Behälter gegeben, der vorteilhaft bereits eine Menge aufgeschmolzener, in entsprechender Weise gereinigter, Acrylsäurekristalle enthält. Bei Bedarf enthält diese aufgeschmolzene Acrylsäure zusätzlichen Polymerisationsinhibitor zugesetzt (z.B. MEHQ, PTZ oder 4-Hydroxy-TEMPO) . In der Regel genügt jedoch der in den Kristallen verbliebene Inhibitorrest, um eine ausreichende Inhibierung zu gewährleisten. Durch indirektes Erwärmen werden anschließend die gewonnenen Acrylsäurekristalle aufgeschmolzen. Die so erhältliche Acrylsäureschmelze weist normalerweise eine Reinheit >98 Gew. -% auf und kann unmittelbar als veresterungsgerechte Acrylsäure vermarktet werden.Acrylic acid crystals as a solid phase containing suspension. A crystallizer that is particularly suitable in this regard is a crystallizer from GMF Gouda (Holland) of the type cooling disc crystallizer. The crystals formed are expediently separated from the abovementioned suspension by means of a centrifuge (for example one from Siebtechnik, of the SHS type centrifugal centrifuge with conical sieve drum) and, if necessary, washed with crude acrylic acid which has already been further purified and / or sweating described later subject. The separation and washing of the suspension crystals can, however, also advantageously be carried out in a washing column, as it is e.g. in EP-A 97405, US-A 3872009, EP-A 98637, EP-A 305316, EP-A 105524 and WO 84/00118. Then the crystals are usually placed in a container which advantageously already contains a quantity of melted, appropriately cleaned, acrylic acid crystals. If necessary, this melted acrylic acid contains additional polymerization inhibitor (e.g. MEHQ, PTZ or 4-hydroxy-TEMPO). As a rule, however, the inhibitor residue remaining in the crystals is sufficient to ensure adequate inhibition. The acrylic acid crystals obtained are then melted by indirect heating. The acrylic acid melt obtainable in this way normally has a purity> 98% by weight and can be marketed directly as esterification-compatible acrylic acid.
Anstelle einer Suspensionskristallisation kann auch eine Schicht - kristallisation, z.B. eine Fallfilmkristallisation, angewendet werden, wie sie z.B. in der EP-A 616 998. zur Gewinnung von Rei- nacrylsäure beschrieben ist. Als flüssiger Kälte-/Wärmeträger kommen dabei z.B. Wasser/Methanol, Wasser/Ethanol- und Wasser/ Ethylenglycolmischungen in Betracht.Instead of suspension crystallization, layer crystallization, e.g. falling film crystallization, as used e.g. is described in EP-A 616 998 for obtaining pure acrylic acid. The liquid coolant / heat transfer medium is e.g. Water / methanol, water / ethanol and water / ethylene glycol mixtures are considered.
Zur Gewinnung besonders hoher Reinheitsgrade ("Reinacrylsäure" ) wird die Kristallisation zweckmäßigerweise als fraktionierte Kri- stallisation durchgeführt. Üblicherweise werden bei fraktionierter Kristallisation alle Stufen, die ein Kristallisat erzeugen, das reiner ist als die zugeführte Schmelze, Reinigungsstufen genannt. Alle anderen Stufen heißen Abtriebsstufen. Zweckmäßiger - weise werden mehrstufige Verfahren nach dem Gegenstromprinzip be- trieben, bei dem nach der Kristallisation in jeder Stufe das Kristallisat von der Mutterlauge abgetrennt und dieses Kristallisat der jeweiligen Stufe mit dem nächsthöheren Reinheitsgrad zuge- führt wird, während der Kristallisationsrückstand der jeweiligen Stufe mit dem nächstniedrigen Reinheitsgrad zugeführt wird.To obtain particularly high degrees of purity (“pure acrylic acid”), the crystallization is expediently carried out as a fractional crystallization. In the case of fractional crystallization, all stages which produce a crystallizate which is purer than the supplied melt are usually called cleaning stages. All other stages are called output stages. Appropriately - multi-stage processes are operated according to the countercurrent principle, in which after crystallization the crystals are separated from the mother liquor in each stage and this crystals are added to the respective stage with the next highest degree of purity is carried out while the crystallization residue is fed to the respective stage with the next lowest degree of purity.
Vorteilhafterweise liegt die Temperatur der Flüssigphase während der Kristallisation zwischen +5 und + 14°C, insbesondere zwischen +8°C und +12°C. Der Feststoffgehalt im Kristallisator liegt vorteilhafterweise zwischen 0 und 80 g Feststoff/100 g Gesamtmasse. Bei der Suspensionskristallisation beträgt der Feststoffgehalt vorzugsweise zwischen 15 und 35 g Feststoff/100 g Gesamtmasse und bei der Schichtkristallisation 50 bis 80 g Feststoff/100 g Gesamtmasse.The temperature of the liquid phase during the crystallization is advantageously between +5 and + 14 ° C, in particular between + 8 ° C and + 12 ° C. The solids content in the crystallizer is advantageously between 0 and 80 g solids / 100 g total mass. In the case of suspension crystallization, the solids content is preferably between 15 and 35 g of solids / 100 g of total mass and in the case of layer crystallization 50 to 80 g of solids / 100 g of total mass.
In einer möglichen Ausgestaltung der Erfindung erfolgt die Kristallisation durch Kühlung von Apparatewänden und/oder durch Ver- dampfung der Lösung im Vakuum. Bei der Kristallisation durch Kühlung wird die Wärme über Kratzkühler, die mit einem Rührkessel oder einem Behälter ohne Rührwerk verbunden sind, abgeführt. Der Umlauf der Kristallsuspension wird hierbei durch eine Pumpe gewährleistet. Daneben besteht auch die Möglichkeit, die Wärme über die Wand eines Rührkessels mit wandgängigem Rührer abzuführen. Eine weitere Ausführungsform bei der Kühlungskristallisation ist die Verwendung von Kühlscheibenkristallern, wie sie z.B. von der Fa. Gouda (Holland) hergestellt werden. Bei einer weiteren geeigneten Variante zur Kristallisation durch Kühlung wird die Wärme über herkömmliche Wärmeüberträger (bevorzugt Rohrbündel oderIn one possible embodiment of the invention, the crystallization takes place by cooling the apparatus walls and / or by evaporating the solution in vacuo. In the case of crystallization by cooling, the heat is dissipated via scratch coolers which are connected to a stirred kettle or a container without a stirrer. The circulation of the crystal suspension is guaranteed by a pump. In addition, there is also the option of dissipating the heat through the wall of a stirred tank with a wall-mounted stirrer. Another embodiment in cooling crystallization is the use of cooling disk crystallizers, such as are used e.g. manufactured by Gouda (Holland). In a further suitable variant for crystallization by cooling, the heat is transferred via conventional heat exchangers (preferably tube bundle or
Plattenwärmeüberträger) abgeführt. Diese Apparate besitzen im Gegensatz zu Kratzkühlern, Rührkesseln mit wandgängigen Rührern oder Kühlkristallscheiben keine Vorrichtung zur Vermeidung von Kristallschichten auf den Wärmeübertragenden Flächen. Wird im Be- trieb ein Zustand erreicht, bei dem der Wärmedurchgangswiderstand durch Kristallschichtbildung einen zu hohen Wert annimmt, erfolgt die Umschaltung auf einen zweiten Apparat. Während der Betriebs- zeit des zweiten Apparats wird der erste Apparat regeneriert (vorzugsweise durch Abschmelzen der Kristallschicht oder Durch- spülen des Apparats mit aufgeschmolzenen Kristallen) . Wird im zweiten Apparat ein zu hoher Wärmedurchgangswiderstand erreicht, schaltet man wieder auf den ersten Apparat um usw. Diese Variante kann auch mit mehr als zwei Apparaten im Wechsel betrieben werden. Außerdem kann die Kristallisation durch eine herkömmliche Verdampfung der Lösung im Vakuum erfolgen. In einer weiteren Ausführung der Erfindung erfolgt die Kristallisation in Apparaten, in denen die Kristalle im Kristallisationsapparat an gekühlten Flächen aufwachsen, d.h. im Apparat fixiert sind (z.B. Schicht - kristallisationsverfahren (vgl. z.B. EP-A 616998) der Fa. Sulzer Chemtech (Schweiz) oder statisches Kristallisationsverfahren, (vgl. z.B. FR-A 2668946) der Fa. BEFS PROKEM (Frankreich)). Die erhaltenen Acrylsäurekristalle werden wie bereits erwähnt von der verbliebenen Mutterlauge abgetrennt. Für den Fall der Schichtkristallisation oder der statischen Kristallisation kann die Trennung der Kristalle von der Mutterlauge im Kristallisat! - onsapparat selbst erfolgen, da die Kristalle im Apparat fixiert sind und die Mutterlauge durch Abfließenlassen aus dem Apparat entfernt werden kann. Die Entfernung der Kristalle aus dem Kristallisationsapparat erfolgt durch Aufschmelzen der Kristalle und nachfolgendes Abfließenlassen der Schmelze. Für den Fall der Sus- pensionskristallisation eignen sich alle bekannten Verfahren der Fest-Flüssig-Trennung. In einer bevorzugten Ausführungsform der Erfindung werden die Kristalle durch Filtrieren und/oder Zentri- fugieren von der Mutterlauge abgetrennt. Vorteilhafterweise wird dem Filtrieren oder Zentrifugieren eine Voreindickung der Suspen- sion, zum Beispiel durch Hydrozyklon (e) , vorgeschaltet. Zum Zentrifugieren eignen sich alle bekannten Zentrifugen, die diskontinuierlich oder kontinuierlich arbeiten. Am vorteilhaftesten werden Schubzentrifugen verwendet, die ein- oder mehrstufig betrieben werden können. Daneben eignen sich auch Schneckensieb- Zentrifugen oder Schneckenaustragszentrifugen (Dekanter) . Eine Filtration erfolgt vorteilhafterweise mittels Filternutschen, die diskontinuierlich oder kontinuierlich, mit oder ohne Rührwerk, oder mittels Bandfilter betrieben werden. Allgemein kann das Filtrieren unter Druck oder im Vakuum erfolgen.Plate heat exchanger) dissipated. In contrast to scratch coolers, stirred kettles with wall-mounted stirrers or cooling crystal disks, these devices have no device for avoiding crystal layers on the heat-transferring surfaces. If a state is reached in operation in which the thermal resistance takes on too high a value due to crystal layer formation, the switchover to a second apparatus takes place. During the operating time of the second apparatus, the first apparatus is regenerated (preferably by melting the crystal layer or flushing the apparatus with melted crystals). If the heat transfer resistance in the second set is too high, switch back to the first set, etc. This variant can also be operated in alternation with more than two sets. In addition, the crystallization can be carried out by conventional evaporation of the solution in vacuo. In a further embodiment of the invention, the crystallization takes place in apparatuses in which the crystals grow on cooled surfaces in the crystallization apparatus, ie are fixed in the apparatus (for example layer crystallization processes (cf. for example EP-A 616998) from Sulzer Chemtech (Switzerland) or static crystallization process (see, for example, FR-A 2668946) from BEFS PROKEM (France)). The acrylic acid crystals obtained are, as already mentioned, separated from the remaining mother liquor. In the case of layer crystallization or static crystallization, the separation of the crystals from the mother liquor in the crystals! - on the apparatus itself, since the crystals are fixed in the apparatus and the mother liquor can be removed from the apparatus by draining off. The crystals are removed from the crystallizer by melting the crystals and then allowing the melt to flow away. In the case of suspension crystallization, all known methods of solid-liquid separation are suitable. In a preferred embodiment of the invention, the crystals are separated from the mother liquor by filtering and / or centrifuging. The filtering or centrifuging is advantageously preceded by a pre-thickening of the suspension, for example by means of hydrocyclone (s). All known centrifuges that work discontinuously or continuously are suitable for centrifugation. Shear centrifuges which can be operated in one or more stages are most advantageously used. In addition, screw screen centrifuges or screw discharge centrifuges (decanters) are also suitable. Filtration is advantageously carried out by means of suction filters, which are operated batchwise or continuously, with or without an agitator, or by means of a belt filter. In general, the filtering can be carried out under pressure or in vacuo.
Während und/oder nach der Fest-Flüssig-Trennung können weitere Verfahrensschritte zur Steigerung der Reinheit der Kristalle bzw. des Kristallkuchens vorgesehen werden. In einer besonders vorteilhaften Ausgestaltung der Erfindung schließt sich nach dem Ab- trennen der Kristalle von der Mutterlauge ein ein- oder mehrstufiges Waschen und/oder Schwitzen der Kristalle oder des Kristall - kuchens an. Beim Waschen liegt die Waschflüssigkeitsmenge geeigneterweise zwischen 0 und 500 g Waschflüssigkeit/100 g Kristallisat, vorzugsweise zwischen 30 und 200 g Waschflüssigkeit/100 g Kristallisat. Die verwendete Waschflüssigkeit unterliegt keiner Einschränkung. Vorteilhafterweise wird jedoch mit Reinprodukt gewaschen, d.h. mit einer Flüssigkeit, die Acrylsäure enthält, deren Reinheit gleich oder höher ist als die des zu waschenden Kristallkuchens. Daneben ist auch eine Wäsche mit Wasser möglich. Das Waschen kann in hierfür üblichen Apparaten erfolgen.During and / or after the solid-liquid separation, further process steps can be provided to increase the purity of the crystals or the crystal cake. In a particularly advantageous embodiment of the invention, after the crystals have been separated from the mother liquor, the crystals or the crystal cake are washed and / or sweated in one or more stages. When washing, the amount of washing liquid is suitably between 0 and 500 g of washing liquid / 100 g of crystals, preferably between 30 and 200 g of washing liquid / 100 g of crystals. The washing liquid used is not subject to any restrictions. However, it is advantageous to wash with pure product, i.e. with a liquid containing acrylic acid, the purity of which is equal to or higher than that of the crystal cake to be washed. Washing with water is also possible. Washing can be carried out in the usual apparatus for this.
Vorteilhafterweise werden Waschkolonnen (z.B. solche, mit mechanischem Betttransport (z.B. von der Fa. Niro Process Technology B.V. , in s'Hertogenbusch (NL) ) oder solche mit hydraulischem Betttransport (z.B. von der Firma TNO in Apeldoorn (NL) ) ) , in denen die Abtrennung der Mutterlauge und das Waschen in einem Apparat erfolgen, Zentrifugen, die ein- oder mehrstufig betrieben werden können, oder Filternutschen oder Bandfilter verwendet. Das Waschen kann auf Zentrifugen oder Bandfiltern ein- oder mehrstufig durchgeführt werden. Hierbei kann die Waschflüssigkeit im Gegenstrom zum Kristallkuchen geführt werden.Washing columns (e.g. those with mechanical bed transport (e.g. from Niro Process Technology BV, in s'Hertogenbusch (NL)) or those with hydraulic bed transport (e.g. from TNO in Apeldoorn (NL))) are advantageous in which the separation of the mother liquor and the washing take place in one apparatus, centrifuges which operate in one or more stages can be used, or filter suction or band filter. Washing can be carried out on centrifuges or belt filters in one or more stages. Here, the washing liquid can be passed in countercurrent to the crystal cake.
Beim Schwitzen handelt es sich um ein lokales Abschmelzen verunreinigter Bereiche. Vorteilhafterweise beträgt die Schwitzmenge zwischen 0 und 80 g abgeschmolzenes Kristallisat/100 g Kristallisat vor dem Schwitzen, vorzugsweise zwischen 5 und 35 g abge- schmolzenes Kristallisat/100 g Kristallisat. Besonders bevorzugt ist die Durchführung des Schwitzens auf Zentrifugen oder Bandfiltern sowie in Kristallern, in denen die Kristalle im Kristaller fixiert sind (z.B. Schichtkristaller) . Auch die Durchführung einer Kombination aus Waschen und Schwitzen in einem Apparat kann geeignet sein.Sweating is a local melting of contaminated areas. The amount of sweat is advantageously between 0 and 80 g of melted crystals / 100 g of crystals before sweating, preferably between 5 and 35 g of melted crystals / 100 g of crystals. It is particularly preferred to carry out the sweating on centrifuges or belt filters and in crystallizers in which the crystals are fixed in the crystallizer (e.g. layer crystallizer). Carrying out a combination of washing and sweating in one apparatus can also be suitable.
Die Acrylsäurekristalle nach der Fest-Flüssig-Trennung und ggf. weiterem Waschen und/oder Schwitzen stellen die gereinigte Säure aus dem Verfahren dar. Die Reinheit der erhaltenen Kristalle be- trägt in der Regel 97 bis 99,99 Gew. -% und mehr Acrylsäure, insbesondere 98,5 bis 99,9 Gew.-% Acrylsäure. Die nach dem erfindungsgemäßen Verfahren hergestellten Kristalle enthalten nurmehr ganz geringe Mengen an Verunreinigungen, wie Essigsäure, Propion- säure und/oder Diacrylsäure.The acrylic acid crystals after the solid-liquid separation and possibly further washing and / or sweating represent the purified acid from the process. The purity of the crystals obtained is generally 97 to 99.99% by weight or more acrylic acid , in particular 98.5 to 99.9% by weight of acrylic acid. The crystals produced by the process according to the invention now contain only very small amounts of impurities, such as acetic acid, propionic acid and / or diacrylic acid.
Zusammenfassend sei nochmals festgehalten, daß die Kristallisation erfindungsgemäß prinzipiell als Suspensionskristallisation und/oder als Schichtkristallisation (die abgeschiedenen Kristalle verbleiben im Kristaller fixiert) realisiert werden kann. Als letztere Kristallisationsmethode kommt z.B. die Fallfilmkristallisation (z.B. wie in EP-A 616 998 beschrieben), die Kristallisation im volldurchströmten Rohr (z.B. gemäß DE-A 2606364) oder eine statische Kristallisation in Betracht.In summary, it should be stated once again that the crystallization according to the invention can in principle be carried out as suspension crystallization and / or as layer crystallization (the deposited crystals remain fixed in the crystallizer). The latter crystallization method comes e.g. falling film crystallization (e.g. as described in EP-A 616 998), crystallization in a fully flow-through tube (e.g. according to DE-A 2606364) or static crystallization.
Bei der Suspensionskristallisation kann die Kühlung direkt (z.B. Verdampfung im Vakuum) und/oder indirekt mittels gekühlter Flächen redisiert werden. Als Kristaller kommen für eine solche Suspensionskristallisation in Betracht: Rührkessel mit wandgängigen Rührern, Kratzkühler, Kühlscheibenkristaller der Fa. Gouda, sowie Umlaufkristallisation mit Wärmeüberträgern ohne Vorrichtung zur Vermeidung von Kristallschichtbildung.In the case of suspension crystallization, the cooling can be redirected directly (e.g. evaporation in a vacuum) and / or indirectly using cooled surfaces. Possible crystallizers for such a suspension crystallization are: stirred tanks with wall-mounted stirrers, scratch coolers, cooling disk crystallizers from Gouda, and circulation crystallization with heat exchangers without a device for avoiding crystal layer formation.
Falls gewünscht, kann die gereinigte Säure nach bekannten Methoden verestert oder nach bekannten Methoden weiter gereinigt wer- den. Zur Erhöhung der Ausbeute an Acrylsäure wird ganz generell die nach Beendigung der Kristallisation verbliebene Mutterlauge wenigstens teilweise, wie eingangs beschrieben, in die Trennkolonne rückgeführt. Der Anteil der rückgeführten Mutterlauge liegt, be- zogen auf ihre anfallende Menge, erfindungsgemäß bei > 0 bisIf desired, the purified acid can be esterified by known methods or further purified by known methods. To increase the yield of acrylic acid, the mother liquor which remains after the crystallization has ended is, at least in part, recycled into the separation column, as described at the outset. The proportion of the recirculated mother liquor, based on its amount, is> 0 to, according to the invention
100 Gew.-%, vorzugsweise bei 80 bis 100 Gew.-%. Über die Rückführung der Mutterlauge wird gleichzeitig die Rückführung des bei der Kristallisation abgetrennten Polymerisationsinhibitors gewährleistet. Das erfindungsgemäße Verfahren gewährleistet eine voll befriedigende Polymerisationsinhibierung bei minimiertem Einsatz an Polymerisationsinhibitoren.100% by weight, preferably 80 to 100% by weight. By recycling the mother liquor, the recycling of the polymerization inhibitor separated off during the crystallization is ensured at the same time. The process according to the invention ensures a completely satisfactory inhibition of polymerization with minimized use of polymerization inhibitors.
Bei Bedarf kann erfindungsgemäß zusätzlich zum Produktgemisch noch in begleitender Weise molekularer Sauerstoff oder ein mole- kularen Sauerstoff enthaltender Inertgasstrom durch die Trennkolonne geführt werden. Dies verstärkt die Wirkung der zugesetzten Polymerisationsinhibitoren .If necessary, in addition to the product mixture, molecular oxygen or an inert gas stream containing molecular oxygen can be passed through the separation column in an accompanying manner. This enhances the effect of the polymerization inhibitors added.
In entsprechender Weise wie hier beschrieben, läßt sich selbstre- dend das zum beschriebenen Verfahren äquivalente Verfahren zur Herstellung von Methacrylsäure inhibieren und betreiben.In a corresponding manner as described here, the method for the production of methacrylic acid equivalent to the described method can be inhibited and operated.
Mögliche Ausgangsverbindungen für die Gasphasenoxidation sind dabei Isobuten, Methyl -tert.butylether, Isobutan, Isobuttersäure, tert. Butanol, Isobutyraldehyd oder Methacrolein. Im übrigen gelten diesbezüglich die in der DE-A 19740253 und DE-A 19740252 gemachten Angaben.Possible starting compounds for the gas phase oxidation are isobutene, methyl tert-butyl ether, isobutane, isobutyric acid, tert. Butanol, isobutyraldehyde or methacrolein. Otherwise, the information given in DE-A 19740253 and DE-A 19740252 apply in this regard.
Wie beschrieben kann die Polymerisationsinhibierung beim erfindungsgemäßen Verfahren auch so vorgenommen werden, daß man am Kopf der Kolonne eine wäßrige MEHQ-Lösung aufgibt und in den Mittelteil der Kolonne eine Lösung von PTZ in Acrylsäure zusetzt. Der Sauerwasserquench wird dann ebenfalls mittels wäßriger MEHQ- Lösung stabilisiert.As described, the polymerization inhibition in the process according to the invention can also be carried out by adding an aqueous MEHQ solution to the top of the column and adding a solution of PTZ in acrylic acid to the middle part of the column. The acidic water quench is then also stabilized using an aqueous MEHQ solution.
Es überrascht, daß die erfindungsgemäße Integration der Rückspaltung die Standzeit des Aufarbeitungsverfahrens so gut wie nicht beeinträchtigt Fig. 2 zeigt eine schematische Abbildung der erfindungsgemäßen Verfahrensweise. Die Bedeutung der Ziffern in Fig. 2 weisen das Vergleichsbeispiel und die Beispiele aus. BeispieleIt is surprising that the integration of the cleavage according to the invention has practically no effect on the service life of the reprocessing process. The meaning of the numbers in FIG. 2 show the comparative example and the examples. Examples
Vergleichsbeispiel (die in diesem Beispiel verwendeten Bezugszif fern beziehen sich auf die dieser Schrift beiliegende Figur 1)Comparative example (the reference numerals used in this example relate to FIG. 1 enclosed with this document)
Aus einer heterogen katalysierten Gasphasenoxidation wurde ein eine Temperatur von 270°C aufweisendes Produktgasgemisch (1) der nachfolgenden Zusammensetzung erhalten:A product gas mixture (1) of the following composition, which had a temperature of 270 ° C., was obtained from a heterogeneously catalyzed gas phase oxidation:
11,5 Gew. -% Acrylsäure,11.5% by weight of acrylic acid,
0,3 Gew.-% Essigsäure,0.3% by weight of acetic acid,
30 Gew.ppm Propionsäure,30 ppm by weight propionic acid,
0,09 Gew. -% Maleinsäureanhydrid,0.09% by weight of maleic anhydride,
0,01 Gew.-% Acrolein, 0,1 Gew. -% Formaldehyd,0.01% by weight of acrolein, 0.1% by weight of formaldehyde,
30 Gew.ppm Furfural,30 ppm by weight furfural,
0,001 Gew. -% Benzaldehyd,0.001% by weight of benzaldehyde,
0, 3 Gew. -% Propen,0.3% by weight propene,
3,4 Gew. -% Sauerstoff, 5,3 Gew. - Wasser,3.4% by weight oxygen, 5.3% by weight water,
1,7 Gew.-% Kohlenoxide, und als Restmenge N2 1.7 wt .-% carbon oxides, and as a residual amount of N 2
Das Produktgasgemisch (3600 g/h) wurde in einem Sprühkühler (2) auf eine Temperatur von 136°C abgekühlt. Als Sprühflüssigkeit (Quenchflüssigkeit 1) wurden 750g/h (7) von insgesamt 7000g/h über den Fangboden (5) (mit einer Temperatur von 100°C) aus derThe product gas mixture (3600 g / h) was cooled to a temperature of 136 ° C. in a spray cooler (2). The spray liquid (quench liquid 1) was 750 g / h (7) out of a total of 7000 g / h via the collecting tray (5) (with a temperature of 100 ° C.) from the
Trennkolonne (3) entnommener Schwersiederfraktion (6) verwendetSeparation column (3) taken from the high boiler fraction (6)
(Sumpfflüssigkeit 4 trat nicht auf) . Über den mit Wärmeträgeröl betriebenen Rohrbündelwärmetauscher (8) wurde die Sprühflüssig - keit im Kreis geführt. 40 g/h der Quenchflüssigkeit 1 wurden dem Kreislauf als Auslaß kontinuierlich entnommen (9) .(Bottom liquid 4 did not occur). The spray liquid was circulated via the tube bundle heat exchanger (8) operated with thermal oil. 40 g / h of quench liquid 1 were continuously taken out of the circuit as an outlet (9).
Das auf eine Temperatur von 136°C abgekühlte Produktgasgemisch wurde unterhalb des Fangbodens (5) der Trennkolonne zugeführt (10) .The product gas mixture cooled to a temperature of 136 ° C. was fed to the separation column (10) below the tray (5).
Die Kolonne war eine Bodenkolonne mit 25 Dual -flow- und 50 Glok- kenböden (1 Glocke pro Boden) . Der Bodendurchmesser betrug 49 mm. Die Dual-flow-Böden wiesen 6 Löcher pro Boden auf. Der Lochdurch- messer der ersten fünf Dual -flow-Böden betrug 9,5 mm. Die darauffolgenden 10 Böden hatten einen Lochdurchmesser von 9 mm und der Lochdurchmesser der letzten 5 Dual -flow-Böden betrug 8,7 mm. Der Boden obenhalb von Boden 15 war als weiterer Fangboden (11) gestaltet. Über ihm wurden 1800g/h einer eine Temperatur von 97°C aufweisenden Rohacrylsäure (12) enthaltend Acrylsäure 97,3 Gew.-%The column was a plate column with 25 dual-flow plates and 50 bell plates (1 bell per plate). The bottom diameter was 49 mm. The dual-flow floors had 6 holes per floor. The hole diameter of the first five dual-flow trays was 9.5 mm. The subsequent 10 trays had a hole diameter of 9 mm and the hole diameter of the last 5 dual-flow trays was 8.7 mm. The floor above floor 15 was designed as another collecting floor (11). 1800 g / h of crude acrylic acid (12) having a temperature of 97 ° C. were contained above it Acrylic acid 97.3% by weight
Essigsäure 0,8 Gew. -%Acetic acid 0.8% by weight
Propionsäure 600 Gew.ppmPropionic acid 600 ppm by weight
Furfural 700 Gew.ppm Maleinsäureanhydrid 40 Gew.ppmFurfural 700 ppm by weight maleic anhydride 40 ppm by weight
Benzaldehyd 200 Gew.ppmBenzaldehyde 200 ppm by weight
Wasser 1, 3 Gew. -%Water 1, 3% by weight
abgezogen und einem Suspensionskristaller (13) zugeführt. Ein Teil (6250 g/h) der entnommenen Schwersiederfraktion (14) wurde in einem mit Wärmeträgeröl betriebenen Rohrbündelwärmetauscher auf 105°C erwärmt und auf den 5ten Boden in die Kolonne rückgeführt (16) .withdrawn and fed to a suspension crystallizer (13). A portion (6250 g / h) of the high boiler fraction (14) removed was heated to 105 ° C. in a tube bundle heat exchanger operated with heat transfer oil and returned to the fifth tray in the column (16).
Der Kristaller war ein Rührbehälter (3 1 Innenvolumen) mit Wen- delrührer. Die Kristallisationswärme wurde über den Doppelmantel des Behälters abgeführt. Die Gleichgewichtstemperatur der Lösung betrug 9,7°C. Die bei der Kristallisation erzeugte Suspension (Feststoffgehalt ca. 25 Gew. -%) wurde auf einer Zentrifuge bei 2000 U/min (Zentrifugendurchmesser 300 mm) und einer Schleuder - zeit von 3 min diskontinuierlich in Kristalle und Mutterlauge getrennt. Die Kristalle wurden anschließend mit aufgeschmolzenem (zuvor gewaschenem) Kristallisat (80 g) 20 sec lang bei 2000 U/ min gewaschen. Die Mutterlauge wurde zusammen mit der Wasch- flüssigkeit auf den 15ten Boden in die Trennkolonne rückgeführt (28) .The crystallizer was a stirred tank (3 1 internal volume) with a spiral stirrer. The heat of crystallization was removed via the double jacket of the container. The equilibrium temperature of the solution was 9.7 ° C. The suspension produced during the crystallization (solids content approx. 25% by weight) was discontinuously separated into crystals and mother liquor on a centrifuge at 2000 rpm (centrifuge diameter 300 mm) and a centrifuging time of 3 min. The crystals were then washed with melted (previously washed) crystals (80 g) for 20 seconds at 2000 rpm. The mother liquor, together with the washing liquid, was returned to the 15th tray in the separation column (28).
Die Analyse der aufgeschmolzenen Reinkristalle (370 g/h) ergab folgende Gehalte:The analysis of the melted pure crystals (370 g / h) showed the following contents:
Acrylsäure 99,5 Gew.-%Acrylic acid 99.5% by weight
Essigsäure 0,3 Gew. -%Acetic acid 0.3% by weight
Propionsäure 200 Gew.ppmPropionic acid 200 ppm by weight
Maleinsäureanhydrid 20 Gew.ppm Furfural 20 Gew.ppmMaleic anhydride 20 ppm by weight Furfural 20 ppm by weight
Benzaldehyd 5 Gew.ppmBenzaldehyde 5 ppm by weight
Wasser 0,06 Gew. -%Water 0.06% by weight
Am Kopf der Kolonne wurde ein gasförmiges Gemisch (17) entnommen und im Sprühkühler (18) einer Partialkondensation unterworfen. 480 g/h des dabei anfallenden Sauerwassers wurden am Kopf der Kolonne mit einer Temperatur von 30°C in selbige zurückgeführt (26)- 220 g/h des Sauerwassers wurden kontinuierlich entnommen. 90 g/h des entnommenen Sauerwassers wurden mit MEHQ (22) versetzt und als 0,5 gew. -%ige wässrige Stabilisatorlösung (21) gemeinsam mit der Restmenge des Sauerwassers (23) über den wassergekühlten Rohrbündelwärmetauscher (24) auf 18°C abgekühlt als Sprühflüssig- keit (25) verwendet. Mit einem anderen Teil des entnommenen Sauerwassers (ersatzweise kann hier auch eine entsprechende Menge Frischwasser verwendet werden) wurde eine 0,5 gew. -%ige wäßrige Lösung von 4 -Hydroxy- EMPO hergestellt, die in einer Menge von 18 g/h mit einer Temperatur von 20 bis 30°C auf dem 75ten Boden der Trennkolonne zugeführt wurde (27) .A gaseous mixture (17) was removed from the top of the column and subjected to a partial condensation in the spray cooler (18). 480 g / h of the resulting acid water were returned to the top of the column at a temperature of 30 ° C. (26) - 220 g / h of the acid water were continuously removed. 90 g / h of the sour water removed were mixed with MEHQ (22) and as 0.5 wt. -% aqueous stabilizer solution (21) together with the remaining amount of acid water (23) cooled to 18 ° C as a spray liquid via the water-cooled tube bundle heat exchanger (24) speed (25) used. With another part of the extracted sour water (a corresponding amount of fresh water can be used as an alternative) a 0.5 wt. -% aqueous solution of 4-hydroxy EMPO prepared, which was fed in an amount of 18 g / h at a temperature of 20 to 30 ° C on the 75th tray of the separation column (27).
Die beschriebene Trennvorrichtung konnte 40 Tage ohne nennenswerte Polymerisatbildung betrieben werden.The separation device described could be operated for 40 days without any appreciable polymer formation.
Beispiel 1example 1
In einem beheizbaren Rührreaktor (Doppelmantel) mit 1 1 Innenvolumen wurden 800 g des Auslaß der Quenchflüssigkeit 1 aus dem Verlgeichsbeispiel mit 1 Gew. -% Na2C03 (bezogen auf das Gewicht der Auslaßmenge versetzt und bei einem Druck von 300 mbar auf 180°C erhitzt. Die Spaltprodukte wurden gasförmig über eine mit Raschigringen (5 mm) gefüllte Kolonne (Füllhöhe: 30 cm) kontinuierlich abgetrennt und ohne Zusatz von Polymerisationsinhibitor kondensiert. Innerhalb von 3,5 h Stunden wurden 730 g Spaltprodukt kondensiert. Dies entspricht einer Rückgewinnungsrate von ca. 90 Gew.-%. Entsprechend der gaschromatographischen Analyse bestand das Kondensat außer aus Acrylsäure und Inhibitor noch aus Diacrylsäure (ca. 1 Gew. -%) , 0,15 Gew. -% Benzaldehyd und ca. 0,08 Gew.-% Furfurol .In a heatable stirred reactor (double jacket) with 1 1 internal volume, 800 g of the outlet of the quench liquid 1 from the comparative example were mixed with 1% by weight Na 2 CO 3 (based on the weight of the outlet amount and at a pressure of 300 mbar at 180 ° C. The cleavage products were continuously separated in gaseous form via a column filled with Raschig rings (5 mm) (filling height: 30 cm) and condensed without the addition of a polymerization inhibitor. 730 g of cleavage product were condensed within 3.5 hours, which corresponds to a recovery rate of According to the gas chromatographic analysis, the condensate consisted not only of acrylic acid and inhibitor, but also of diacrylic acid (approx. 1% by weight), 0.15% by weight of benzaldehyde and approx. 0.08% by weight. % Furfural.
Beispiel 2Example 2
Es wurde wie in Beispiel 1 verfahren, doch an Stelle von 1 Gew. -% NaC03 wurde 1 Gew. -% Dodecylbenzolsulfonsäure als Spaltkatalysator eingesetzt. Innerhalb von 4,75 h wurden 495 g Spaltprodukt einer ähnlichen Zusammensetzung wie in Beispiel 1 kondensiert. Dies entspricht einer Rückgewinnungsrate von ca. 61 Gew.-%.The procedure was as in Example 1, but instead of 1% by weight NaCO 3 , 1% by weight dodecylbenzenesulfonic acid was used as the cracking catalyst. 495 g of cleavage product of a composition similar to that in Example 1 were condensed within 4.75 h. This corresponds to a recovery rate of approx. 61% by weight.
Beispiel 3Example 3
Ein aus Glas bestehender Umlaufreaktor (29), der mit einer Heizkerze beheizt wurde, wurde mit 250 g des Auslaß der Quenchflüssigkeit 1 aus dem Vergleichsbeispiel gefüllt und erhitzt. Zusatz - lieh zu dem in den 250 g Auslaß der Quenchflüssigkeit 1 enthaltenen 4 -OH-TEMPO wurden zum Zweck der co-Stabilisierung 2000 gew.ppm Phenothiazin zugesetzt. Die Spalttemperatur betrug 182°C und der Spaltdruck lag bei 500 mbar. Eine Spaltkatalysator- zugäbe (32) erfolgte nicht. Standgeregelt wurden dem wie be- schrieben gefüllten Umlaufreaktor 155 g/h Auslaß der Quenchflüssigkeit 1 (9) zugeführt. Die Spaltprodukte wurden dampfförmig über einen dem Reaktor aufgesetzten Spritzschutz abgetrennt und in einer Menge von 113 g/h (dies entspricht einer Rückgewinnungs - rate von 73 Gew. -%) ohne Zugabe von Polymerisationsinhibitor kondensiert (30) . Das Kondensat bestand neben Acrylsäure und Inhibitor noch aus ca. 3,2 Gew. -% Diacrylsäure, 0,06 Gew. -% Benz - aldehyd, 0,3 Gew. -% Essigsäure und 0,03 Gew. -% Propionsäure.A circulation reactor (29) consisting of glass, which was heated with a heating plug, was filled with 250 g of the outlet of the quench liquid 1 from the comparative example and heated. Additive - loaned to the 4 -OH-TEMPO contained in the 250 g outlet of quench liquid 1, 2000 wt.ppm phenothiazine were added for the purpose of co-stabilization. The gap temperature was 182 ° C and the gap pressure was 500 mbar. A gap catalyst (32) was not added. With level control, 155 g / h of quench liquid 1 (9) were fed to the circulating reactor filled as described. The cleavage products were separated off in vapor form via a splash guard attached to the reactor and condensed in an amount of 113 g / h (this corresponds to a recovery rate of 73% by weight) without addition of a polymerization inhibitor (30). In addition to acrylic acid and inhibitor, the condensate also consisted of approximately 3.2% by weight of diacrylic acid, 0.06% by weight of benzaldehyde, 0.3% by weight of acetic acid and 0.03% by weight of propionic acid.
40 g/h Spaltrückstand wurden dem Spaltreaktor kontinuierlich entnommen (31) und entsorgt.40 g / h of cleavage residue were continuously removed from the cleavage reactor (31) and disposed of.
Beispiel 4Example 4
Wie das Vergleichsbeispiel, der Sprühflüssigkeit wurden jedoch zusätzlich 30 g/h des auf 100°C abgekühltem Rückspaltkondensats aus Beispiel 3 zugeführt (die Zugabestelle lag kurz vor der Sprühdüse) . In entsprechender Weise betrug die Menge der gewönne- nen Reinkristalle ca. 400 g/h.Like the comparative example, the spray liquid was additionally supplied with 30 g / h of the cleavage condensate from Example 3 which had cooled to 100 ° C. (the addition point was just before the spray nozzle). In a corresponding manner, the amount of pure crystals obtained was approximately 400 g / h.
Die Rückführung des Spaltkondensats hatte innerhalb von 40 Betriebstagen der beschriebenen Trennvorrichtung keinen sichtbaren Einfluß auf die Polymerisatbildung. Als alternative Zugabestelle für das Rückspaltkondensat kommt auch die mit R gekennzeichnete Stelle in Fig. 2 in Betracht. The recycling of the split condensate had no visible influence on the polymer formation within 40 days of operation of the separation device described. The location marked with R in FIG. 2 can also be considered as an alternative addition point for the reclassification condensate.

Claims

Patentansprüche claims
1. Verfahren zur Herstellung von Acrylsäure, bei dem man durch heterogen katalysierte Gasphasen-Partialoxidation von Propan oder Propen und/oder Acrolein mit molekularem Sauerstoff an im festen Aggregatzustand befindlichen Katalysatoren bei erhöhter Temperatur ein Acrylsäure enthaltendes heißes Produkt- gasgemisch erzeugt, die Temperatur des heißen, Acrylsäure enthaltenden Produktgasgemisches durch direkte Kühlung mit einer Quenchflüssigkeit 1 verringert, den dabei nicht verdampfenden Teil der Quenchflüssigkeit 1, gegebenenfalls über einen indirekt kühlenden Wärmetauscher, im Kreis führt und als Auslaß einen Teil der nicht verdampften Quenchflüssigkeit 1 aus diesem Kreislauf ausschleust, anschließend das abgekühlte Produktgasgemisch in eine mit trennwirksamen Einbauten ausgerüstete Kolonne leitet, innerhalb der Kolonne in sich selbst aufsteigen läßt und dabei fraktioniert kondensiert, wobei im Seitenabzug eine rohe Acrylsäure und über den Kolon- nensumpf und/oder über einen in Sumpfnähe gelegenen Seitenabzug Acrylsäure-Oligomere enthaltende Sumpfflüssigkeit und/ oder Schwersiederfraktion entnommen und als Quenchflüssigkeit 1 verwendet wird, dadurch gekennzeichnet, daß man den Auslaß der Quenchflüssigkeit 1 einem Spaltgefäß zuführt und in sel- bigem die im Auslaß der Quenchflüssigkeit 1 enthaltenen1. A process for the preparation of acrylic acid, in which a heterogeneously catalyzed gas phase partial oxidation of propane or propene and / or acrolein with molecular oxygen on catalysts in the solid state of aggregation produces a hot product gas mixture containing acrylic acid at elevated temperature, the temperature of the hot , Acrylic acid-containing product gas mixture reduced by direct cooling with a quench liquid 1, the non-evaporating part of the quench liquid 1, optionally via an indirectly cooling heat exchanger, circulates and discharges part of the non-evaporated quench liquid 1 from this circuit as an outlet, then the cooled Passes the product gas mixture into a column equipped with separating internals, allows it to rise within itself in the column and condenses fractionally, with a crude acrylic acid in the side draw and over the column bottom and / or Sump liquid and / or high boiler fraction containing acrylic acid oligomers and removed or used as quench liquid 1 via a side draw close to the sump, characterized in that the outlet of the quench liquid 1 is fed to a cracking vessel and in which the quench liquid 1 contained in the outlet
Acrylsäure-Oligomere bei erhöhter Temperatur in Acrylsäure rückspaltet, dabei gasförmig aus der Flüssigphase entweichende Acrylsäure kondensiert und das resultierende Kondensat in den Kreislauf der Quenchflüssigkeit 1 einspeist.Acrylic acid oligomers are split back into acrylic acid at elevated temperature, condensing gaseous acrylic acid escaping from the liquid phase and feeding the resulting condensate into the circuit of quenching liquid 1.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Rückspaltung im Beisein eines anorganischen Salzes, dessen Zusatz zu einer wäßrigen Lösung einer starken Brönsted- Säure den pH-Wert der wäßrigen Lösung ins alkalische verschiebt, durchgeführt wird.2. The method according to claim 1, characterized in that the cleavage is carried out in the presence of an inorganic salt, the addition of which to an aqueous solution of a strong Bronsted acid shifts the pH of the aqueous solution to alkaline.
3. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß als anorganisches Salz wenigstens eines aus der Gruppe umfassend NaOH, Na2C03, NaHC03 KOH, K2C03, KHC03, LiOH, Li2C03 und CaC03 verwendet wird.3. The method according to claim 2, characterized in that at least one from the group comprising NaOH, Na 2 C0 3 , NaHC0 3 KOH, K 2 C0 3 , KHC0 3 , LiOH, Li 2 C0 3 and CaC0 3 is used as the inorganic salt .
4. Verfahren nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß die Rückspaltung bei einer Temperatur von 130 bis 250°C durchgeführt wird. 4. The method according to any one of claims 1 to 3, characterized in that the cleavage is carried out at a temperature of 130 to 250 ° C.
5. Verfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß die Rückspaltung bei einem Druck von 25 bis 600 mbar durchgeführt wird.5. The method according to any one of claims 1 to 4, characterized in that the cleavage is carried out at a pressure of 25 to 600 mbar.
6. Verfahren nach einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, daß innerhalb der zur fraktionierten Kondensation verwendeten Kolonne 4-OH-TEMPO als Polymerisationsinhibitor mitverwendet wird. 6. The method according to any one of claims 1 to 5, characterized in that 4-OH-TEMPO is used as a polymerization inhibitor within the column used for the fractional condensation.
EP00907643A 1999-03-06 2000-02-28 Method for producing acrylic acid Expired - Lifetime EP1163201B1 (en)

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DE1999109923 DE19909923A1 (en) 1999-03-06 1999-03-06 Production of acrylic acid useful as intermediate e.g. for polymer dispersion, uses N-oxyl radicals in aqueous solution as only polymerization inhibitor in separation of gas phase oxidation product
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DE19924533 1999-05-28
DE1999124533 DE19924533A1 (en) 1999-05-28 1999-05-28 Fractional condensation of acrylic acid-containing oxidation gas with acrylic acid oligomer drawn off and cracked to give acrylic acid which is fed to the quenching liquid circuit
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CN1343195A (en) 2002-04-03
JP4673483B2 (en) 2011-04-20
DE50003359D1 (en) 2003-09-25
WO2000053560A1 (en) 2000-09-14
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BR0008823A (en) 2002-01-15

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